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/mcp/tools/projects.py

@@ -17,6 +17,7 @@ keeps working.
 from __future__ import annotations
 
 from scribe.mcp._context import current_user_id
+from scribe.services import design_systems as design_systems_svc
 from scribe.services import milestones as milestones_svc
 from scribe.services import notes as notes_svc
 from scribe.services import projects as projects_svc
@@ -53,7 +54,13 @@ async def enter_project(project_id: int) -> dict:
 
     Returns a dict with keys: project, milestone_summary, applicable_rules,
     project_rules, subscribed_rulebooks, applicable_rules_truncated,
-    open_tasks, recent_notes.
+    open_tasks, recent_notes, design_system.
+
+    `design_system` is null unless the project points at one. When present it
+    carries the chain-merged guidance (the house style AND this project's
+    departures from it) plus a summary of the token set — treat it as binding
+    for any UI you write, and pull the values with resolve_design_system or
+    get_design_system_stylesheet before reaching for a literal.
     """
     uid = current_user_id()
     project = await projects_svc.get_project(uid, project_id)
@@ -74,9 +81,17 @@ async def enter_project(project_id: int) -> dict:
         uid, is_task=False, project_id=project_id,
         sort="updated_at", limit=5,
     )
+    # A project need not have one, and most installs won't — null is ordinary
+    # here, not a missing prerequisite.
+    design_system = None
+    if project.design_system_id:
+        design_system = await design_systems_svc.design_context(
+            uid, project.design_system_id,
+        )
 
     return {
         "project": project.to_dict(),
+        "design_system": design_system,
         "milestone_summary": milestone_summary,
         "applicable_rules": applicable["rules"],
         "project_rules": applicable.get("project_rules", []),