feat(design-systems): REST + MCP surfaces, at parity

Milestone #254 step 4 (#2290). Eleven capabilities, both surfaces, one service.

Design systems are owner-scoped top-level records rather than project-scoped
ones, so these do not nest under /api/projects/ the way systems do —
routes/rulebooks.py was the closer shape. The one exception is the project
pointer, which is genuinely about a project: PUT /api/projects/<id>/design-system,
PUT rather than PATCH because clearing it is a first-class outcome and not an
omission.

`/resolved` and `/tokens` are deliberately separate endpoints. One answers "what
does this system CHANGE", the other "what does it end up BEING", and a system
that overrides nothing has an empty token list and a full resolved set. Shipping
only one would have made the other a client-side computation of exactly the kind
the record model exists to remove.

ResolvedToken.to_dict carries the SHADOWED contributions, not just the winner.
Dropping them at the serialisation boundary would have discarded the one thing
step 2 was built to preserve, and it would have been invisible — the payload
still looks complete.

Three sentinel translations on the MCP side, each tested, because an agent
cannot omit an argument and a wrong mapping here is silent:

  - parent_id: 0 = unchanged, -1 = clear (become a family system), positive =
    set. Renaming a system must not silently re-root it.
  - order_index: -1 = unchanged, since 0 is a valid position.
  - value_by_mode: guarded on `is not None`, not truthiness, so `{}` can strip
    every mode from a token instead of being unreachable.

DesignSystemCycle maps to 400 on REST and to a ValueError carrying the message
on MCP — kept apart from 404 throughout. An agent told "not found" retries the
same call; one told what the loop is can fix it.

Two structural guards beyond the parity list: every endpoint must be reachable
on the app (catching a decorator copied without its path, where the second
handler silently never runs), and every public coroutine in the tools module
must be registered (a tool written but never registered is invisible to an
agent, and nothing else would notice).

src/scribe/mcp/tools/design_systems.py

@@ -0,0 +1,266 @@
+"""Design system + token MCP tools — wrappers over services/design_systems.py.
+
+A design system is a stylesheet held as records: a named set of tokens with an
+optional parent, so a family system carries the house style and an app system
+carries only what it changes. Precedence by name along the parent chain IS the
+CSS cascade, which is why "what does this app alter?" is a plain list rather
+than a diff.
+
+Parity with the REST surface is a rule, not a nicety (see
+`tests/test_routes_design_systems.py`): an agent and a browser are two callers
+of one service.
+
+Sentinels, matching the milestone/task tool conventions:
+  - title="" / description="" / etc. -> "leave unchanged" on update
+  - parent_id / design_system_id: 0 = leave unchanged, -1 = clear, positive = set
+    (three states, because clearing a parent is a real operation and not the
+    same as omitting the argument)
+  - order_index=-1 -> "leave unchanged" (0 is a valid order_index)
+"""
+from __future__ import annotations
+
+from scribe.mcp._context import current_user_id
+from scribe.services import design_systems as ds_svc
+from scribe.services.design_systems import DesignSystemCycle
+
+
+async def create_design_system(
+    title: str,
+    description: str = "",
+    parent_id: int = 0,
+) -> dict:
+    """Create a design system, optionally inheriting from another.
+
+    Args:
+        title: What this system is, e.g. "FabledSword" or "Scribe" (required).
+        description: What it covers and when it applies.
+        parent_id: Inherit from this system — it holds the defaults this one
+            overrides. Omit (0) for a top-level "family" system, which is what
+            a first design system usually is.
+    """
+    uid = current_user_id()
+    system = await ds_svc.create_design_system(
+        uid,
+        title=title,
+        description=description or None,
+        parent_id=parent_id or None,
+    )
+    if system is None:
+        raise ValueError(f"parent design system {parent_id} not found or not writable")
+    return system.to_dict()
+
+
+async def list_design_systems() -> dict:
+    """List your design systems. An empty list is normal — most installs have none."""
+    uid = current_user_id()
+    rows = await ds_svc.list_design_systems(uid)
+    return {"design_systems": [s.to_dict() for s in rows]}
+
+
+async def get_design_system(design_system_id: int) -> dict:
+    """Fetch a design system plus its OWN tokens — i.e. what it changes.
+
+    For what it actually resolves to once inheritance is applied, use
+    `resolve_design_system`. The two answer different questions and a system
+    that overrides nothing has an empty token list but a full resolved set.
+    """
+    uid = current_user_id()
+    system = await ds_svc.get_design_system(uid, design_system_id)
+    if system is None:
+        raise ValueError(f"design system {design_system_id} not found")
+    tokens = await ds_svc.list_tokens(uid, design_system_id)
+    return {
+        "design_system": system.to_dict(),
+        "tokens": [t.to_dict() for t in tokens],
+    }
+
+
+async def resolve_design_system(design_system_id: int) -> dict:
+    """The EFFECTIVE token set — everything inherited, with this system's on top.
+
+    Each token carries `origin_by_mode` (which system supplied each mode's
+    value) and `contributions` (every system that offered one, deepest first —
+    so entry 0 won and the rest were shadowed). Reach for this when you need to
+    know what a value actually IS; reach for `get_design_system` when you need
+    to know what this system CHANGES.
+    """
+    uid = current_user_id()
+    resolved = await ds_svc.resolve_design_system(uid, design_system_id)
+    if resolved is None:
+        raise ValueError(f"design system {design_system_id} not found")
+    return {
+        "design_system_id": design_system_id,
+        "tokens": [t.to_dict() for t in resolved],
+    }
+
+
+async def update_design_system(
+    design_system_id: int,
+    title: str = "",
+    description: str = "",
+    parent_id: int = 0,
+) -> dict:
+    """Update a design system.
+
+    Args:
+        design_system_id: The system to update.
+        title: New title, or "" to leave unchanged.
+        description: New description, or "" to leave unchanged.
+        parent_id: 0 = leave unchanged, -1 = clear (make this a top-level
+            family system), positive = inherit from that system. A parent that
+            already inherits from this system is refused — that would be a loop.
+    """
+    uid = current_user_id()
+    fields: dict = {}
+    if title:
+        fields["title"] = title
+    if description:
+        fields["description"] = description
+    if parent_id:
+        fields["parent_id"] = None if parent_id == -1 else parent_id
+    try:
+        system = await ds_svc.update_design_system(uid, design_system_id, **fields)
+    except DesignSystemCycle as exc:
+        raise ValueError(str(exc)) from exc
+    if system is None:
+        raise ValueError(f"design system {design_system_id} not found or not writable")
+    return system.to_dict()
+
+
+async def delete_design_system(design_system_id: int) -> dict:
+    """Soft-delete a design system (recoverable).
+
+    Systems that inherited from it become top-level systems keeping their own
+    tokens — deleting a family does not delete the apps under it.
+    """
+    uid = current_user_id()
+    if not await ds_svc.delete_design_system(uid, design_system_id):
+        raise ValueError(f"design system {design_system_id} not found or not writable")
+    return {"message": f"Design system {design_system_id} deleted."}
+
+
+# ── Tokens ──────────────────────────────────────────────────────────────
+
+async def create_design_token(
+    design_system_id: int,
+    name: str,
+    value_by_mode: dict | None = None,
+    group_name: str = "",
+    purpose: str = "",
+    order_index: int = 0,
+) -> dict:
+    """Add a token to a design system.
+
+    Args:
+        design_system_id: The system that owns this token.
+        name: The custom-property name, e.g. "--fs-obsidian" (required).
+        value_by_mode: Values keyed by mode, e.g.
+            {"base": "#f7f5ef", "dark": "#14171a"}. Use "base" for the value
+            that applies when no mode is more specific; a token that is not
+            mode-dependent needs only "base". In a system WITH a parent, an
+            omitted mode is inherited rather than blanked.
+        group_name: Free-text grouping — "surface", "text", "radius", whatever
+            this system's own vocabulary is.
+        purpose: What the token is for, e.g. "page bg, deepest surface".
+        order_index: Display position within its group.
+    """
+    uid = current_user_id()
+    token = await ds_svc.create_token(
+        uid,
+        design_system_id=design_system_id,
+        name=name,
+        value_by_mode=value_by_mode,
+        group_name=group_name or None,
+        purpose=purpose or None,
+        order_index=order_index,
+    )
+    if token is None:
+        raise ValueError(f"design system {design_system_id} not found or not writable")
+    return token.to_dict()
+
+
+async def list_design_tokens(design_system_id: int) -> dict:
+    """A design system's OWN tokens — its override set, not its effective set."""
+    uid = current_user_id()
+    rows = await ds_svc.list_tokens(uid, design_system_id)
+    return {"tokens": [t.to_dict() for t in rows]}
+
+
+async def update_design_token(
+    token_id: int,
+    name: str = "",
+    value_by_mode: dict | None = None,
+    group_name: str = "",
+    purpose: str = "",
+    order_index: int = -1,
+) -> dict:
+    """Update a token. Empty/None args leave a field unchanged.
+
+    `value_by_mode` REPLACES the whole map rather than merging into it, so send
+    every mode you want the token to keep.
+    """
+    uid = current_user_id()
+    fields: dict = {}
+    if name:
+        fields["name"] = name
+    if value_by_mode is not None:
+        fields["value_by_mode"] = value_by_mode
+    if group_name:
+        fields["group_name"] = group_name
+    if purpose:
+        fields["purpose"] = purpose
+    if order_index >= 0:
+        fields["order_index"] = order_index
+    token = await ds_svc.update_token(uid, token_id, **fields)
+    if token is None:
+        raise ValueError(f"design token {token_id} not found or not writable")
+    return token.to_dict()
+
+
+async def delete_design_token(token_id: int) -> dict:
+    """Soft-delete a token (recoverable).
+
+    In a system with a parent this restores inheritance: the token stops being
+    overridden here and resolves to the parent's value again.
+    """
+    uid = current_user_id()
+    if not await ds_svc.delete_token(uid, token_id):
+        raise ValueError(f"design token {token_id} not found or not writable")
+    return {"message": f"Design token {token_id} deleted."}
+
+
+async def set_project_design_system(project_id: int, design_system_id: int = 0) -> dict:
+    """Point a project at a design system.
+
+    Args:
+        project_id: The project to style.
+        design_system_id: The system it uses, or -1 to clear it. Pointing at a
+            system only requires READ access to it — consuming a design system
+            is not changing it.
+    """
+    uid = current_user_id()
+    target = None if design_system_id == -1 else design_system_id
+    ok = await ds_svc.set_project_design_system(uid, project_id, target)
+    if not ok:
+        raise ValueError(
+            f"project {project_id} not writable, or design system "
+            f"{design_system_id} not found"
+        )
+    return {"project_id": project_id, "design_system_id": target}
+
+
+def register(mcp) -> None:
+    for fn in (
+        create_design_system,
+        list_design_systems,
+        get_design_system,
+        resolve_design_system,
+        update_design_system,
+        delete_design_system,
+        create_design_token,
+        list_design_tokens,
+        update_design_token,
+        delete_design_token,
+        set_project_design_system,
+    ):
+        mcp.tool(name=fn.__name__)(fn)