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

tests/test_mcp_tool_projects.py

@@ -17,12 +17,16 @@ def _bind_user():
     _user_id_ctx.reset(token)
 
 
-def _fake_project(**overrides) -> MagicMock:
+def _fake_project(design_system_id=None, **overrides) -> MagicMock:
     p = MagicMock()
     base = {"id": 1, "title": "P", "description": "", "goal": "",
             "status": "active", "color": None}
     base.update(overrides)
     p.to_dict.return_value = base
+    # Explicit, because a bare MagicMock hands back a truthy auto-attribute —
+    # which would route every project in this file through the design-system
+    # branch and out to a real database.
+    p.design_system_id = design_system_id
     return p
 
 
@@ -176,6 +180,45 @@ async def test_enter_project_composes_full_context():
     assert out["open_tasks"][0]["id"] == 100
     assert out["open_tasks"][0]["status"] == "in_progress"
     assert out["recent_notes"][0]["id"] == 200
+    # No design system on this project -> the key is present and null, not
+    # absent. A caller that has to distinguish "no key" from "no system" will
+    # eventually get it wrong.
+    assert out["design_system"] is None
+
+
+@pytest.mark.asyncio
+async def test_enter_project_hands_back_the_design_system_when_the_project_has_one():
+    """The handshake is where an agent learns what binds it, and a design
+    system binds the same way a rule does. Before this it was reachable only by
+    an agent that already knew to call resolve_design_system — so the standards
+    were present in the store and absent from the work."""
+    p = _fake_project(id=5, design_system_id=9)
+    design = {"id": 9, "title": "App kit", "guidance": [{"title": "House"}],
+              "token_count": 95, "token_groups": ["surface"],
+              "inherits_from": ["House"], "description": ""}
+
+    with patch(
+        "scribe.mcp.tools.projects.projects_svc.get_project",
+        AsyncMock(return_value=p),
+    ), patch(
+        "scribe.mcp.tools.projects.rulebooks_svc.get_applicable_rules",
+        AsyncMock(return_value={"rules": [], "truncated": False,
+                                "subscribed_rulebooks": []}),
+    ), patch(
+        "scribe.mcp.tools.projects.milestones_svc.get_project_milestone_summary",
+        AsyncMock(return_value=[]),
+    ), patch(
+        "scribe.mcp.tools.projects.notes_svc.list_notes",
+        AsyncMock(side_effect=[([], 0), ([], 0)]),
+    ), patch(
+        "scribe.mcp.tools.projects.design_systems_svc.design_context",
+        AsyncMock(return_value=design),
+    ) as ctx:
+        out = await enter_project(project_id=5)
+
+    assert out["design_system"]["token_count"] == 95
+    assert out["design_system"]["inherits_from"] == ["House"]
+    assert ctx.await_args.args == (7, 9)   # caller's id, the project's system
 
 
 @pytest.mark.asyncio