feat(design-systems): the master sheet — purpose tokens, not per-element values

Operator's new requirement (#2299, architecture in #2296): a design system does
not just hold tokens, it generates and manages a master CSS sheet. That settles
the milestone's open "authority mechanism" question — the record is
authoritative because the stylesheet comes out of it.

**The sheet is shaped by purpose and styles no elements.** It declares custom
properties, grouped by what they mean, and contains no `.btn-primary`, no
`table`, no `input`. That is the design, not a shortcut: a sheet that styled
elements would restate the same handful of values once per element and grow with
the UI, where purpose-named values are stated once and reused. Components live
as SNIPPETS that reference these names — a surface that already exists and
already carries prose, locations, drift checks, merge and write-path recall.

A token named after an element (`--fs-button-bg`) is the smell that the two have
been mixed; a purpose name (`--fs-action-primary`) is reused across all of them.

Alongside the CSS the endpoint returns what the text cannot say for itself:
which tokens are still valueless, and which VALUES are declared under more than
one name. The second is the operator's "reuse consistent values" constraint made
checkable — and it reports rather than refuses, because a design system
legitimately aligns colours on purpose ("Success = Moss, by design") and only a
human knows which case it is.

Mode maps to selector the way the codebase already does it: base on the root
selector, every other mode layered on `[data-theme="…"]`. The root selector is a
PARAMETER — #251 recorded that a container-scoped preview cannot use `:root`, so
hardcoding it would have made the generator useless to the preview surface.

A token the rulebook names but states no value for is emitted as a commented-out
declaration IN ITS GROUP rather than dropped. Its absence is the finding, and a
comment puts that finding where the reader already is.

Values are validated, not escaped, and this is a real boundary rather than
tidiness: design systems are shareable records (rule #47), so `red; } body {
display: none` in a system shared with you would otherwise inject CSS into your
page. A value containing `{ } ; @ < >`, a comment delimiter or a newline is
REFUSED and rendered as a comment saying so — rejecting beats stripping, since a
partially-sanitised value is one the operator never wrote and the sheet's whole
claim is that it is the record.

Not in scope, and deliberately: serving this as the app's actual stylesheet.
Generating and exposing a sheet is reversible; swapping theme.css for a
generated one is not, and it should be an explicit call rather than a side
effect.

src/scribe/mcp/tools/design_systems.py

@@ -139,6 +139,34 @@ async def delete_design_system(design_system_id: int) -> dict:
     return {"message": f"Design system {design_system_id} deleted."}
 
 
+async def get_design_system_stylesheet(
+    design_system_id: int,
+    root_selector: str = ":root",
+) -> dict:
+    """The master CSS sheet a design system generates.
+
+    Purpose tokens only — this sheet declares what values MEAN and styles no
+    elements. Components (buttons, tables, input schemes) are SNIPPETS that
+    reference these names, so a value is stated once and reused rather than
+    restated per element. Reach for this when you need the tokens a snippet is
+    allowed to use.
+
+    Also returns `valueless` (tokens the system names but has no value for) and
+    `duplicates` (values declared under more than one name — a deliberate alias,
+    or one idea recorded twice).
+
+    Args:
+        design_system_id: The system to render.
+        root_selector: Selector for the base layer. Defaults to `:root`; pass a
+            container selector to scope the sheet to a preview region.
+    """
+    uid = current_user_id()
+    result = await ds_svc.stylesheet_for_system(uid, design_system_id, root_selector)
+    if result is None:
+        raise ValueError(f"design system {design_system_id} not found")
+    return result
+
+
 async def import_design_system_from_rulebook(
     design_system_id: int,
     rulebook_id: int,
@@ -307,6 +335,7 @@ def register(mcp) -> None:
         resolve_design_system,
         update_design_system,
         delete_design_system,
+        get_design_system_stylesheet,
         import_design_system_from_rulebook,
         create_design_token,
         list_design_tokens,