feat(design-systems): give a design system a way to reach the session

Storing a design system never made a session aware of one. Rules get pushed
into every session by the SessionStart hook and returned by enter_project; a
design system had neither, so its standards were reachable only by an agent
that already knew to call resolve_design_system — the same silent failure as a
token nobody declares.

That gap was invisible while the operator's visual standards also lived in a
rulebook. Retiring that rulebook (which is what this unblocks) would have
deleted design guidance from every session with nothing to say so.

- services/design_systems.design_context() — the delivery side. Guidance is
  chain-merged ANCESTOR-FIRST: a child system holds only what it CHANGES, so
  its own guidance describes a departure from a house style it never restates,
  and the leaf alone is a fragment. Tokens are summarised (count + group
  names), not listed — a hundred declarations would crowd out the context they
  are meant to inform.
- enter_project returns `design_system`, null when the project has none.
- The SessionStart context gains a Design system block with pointers to the
  values, alongside the always-on rules.
- server.py's entity list gains Design system, including the negative: do NOT
  record one as a rulebook, because a token kept as prose cannot be resolved,
  inherited, rendered or checked.
- The rulebook-tier passage used "a design-system rulebook" as its worked
  example of a subscribed rulebook — it now teaches the opposite, plus a new
  "is this a rule at all?" test pointing at design systems, processes and
  snippets.
- using-scribe gains a section on building UI against the project's system.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

src/scribe/services/plugin_context.py

@@ -22,6 +22,7 @@ from sqlalchemy import select
 
 from scribe.models import async_session
 from scribe.models.rulebook import RulebookTopic
+from scribe.services import design_systems as design_systems_svc
 from scribe.services import knowledge as knowledge_svc
 from scribe.services import notes as notes_svc
 from scribe.services import projects as projects_svc
@@ -837,6 +838,37 @@ async def build_session_context(
                 f"Goal: {goal[:200]}" if goal else "",
                 f"Open todo tasks: {open_count}",
             ]
+
+            # A design system binds the same way a rule does, and until this
+            # existed it had no push channel — the standards were reachable only
+            # by an agent that already knew to look for them. Summary only: the
+            # token VALUES are a tool call away, and pasting a hundred of them
+            # into every session would crowd out the context they inform.
+            if project.design_system_id:
+                design = await design_systems_svc.design_context(
+                    user_id, project.design_system_id,
+                )
+                if design:
+                    inherits = (
+                        " (inherits " + " › ".join(design["inherits_from"]) + ")"
+                        if design["inherits_from"] else ""
+                    )
+                    groups = ", ".join(design["token_groups"])
+                    lines += [
+                        "",
+                        f"## Design system: {design['title']} "
+                        f"(id {design['id']}){inherits}",
+                        f"{design['token_count']} tokens"
+                        + (f" across {groups}" if groups else "")
+                        + ". This project's UI is built from these, not from "
+                        "literals — reach for a token before writing a colour, "
+                        "size, radius or duration by hand.",
+                        f"Values: `resolve_design_system({design['id']})` · "
+                        f"stylesheet: `get_design_system_stylesheet({design['id']})` "
+                        f"· the prose (aesthetic, voice, where the accent may "
+                        f"appear): `enter_project` returns it, or "
+                        f"`get_design_system({design['id']})`.",
+                    ]
     elif unbound_repo:
         lines += [
             "",