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_services_design_systems.py

@@ -321,3 +321,94 @@ async def test_stylesheet_reports_what_the_sheet_cannot_say_for_itself():
     assert result["duplicates"] == {"#4a5d3f": ["--fs-moss", "--fs-success"]}
     assert "--fs-moss: #4a5d3f;" in result["css"]
     assert "FabledSword" in result["css"]
+
+
+# --- design_context: the delivery side --------------------------------------
+
+def _ctx_token(name: str, group: str | None):
+    from scribe.services.design_cascade import ResolvedToken
+    return ResolvedToken(
+        name=name, contributions={}, group_name=group,
+        purpose=None, rationale=None, supersedes=(), order_index=0,
+    )
+
+
+@pytest.mark.asyncio
+async def test_design_context_merges_guidance_ANCESTOR_FIRST():
+    """LOAD-BEARING. A child system holds only what it CHANGES, so its own
+    guidance describes a departure from a house style it never restates. An
+    agent handed the leaf alone builds against a fragment with no signal that
+    the rest exists — which is exactly the failure retiring the design rulebook
+    would otherwise have caused.
+    """
+    family = MagicMock(id=1, deleted_at=None, owner_user_id=42,
+                       description="the house", guidance="Dark-mode-first.")
+    family.title = "House"
+    app = MagicMock(id=3, deleted_at=None, owner_user_id=42,
+                    description="one app", guidance="Accent on the wordmark.")
+    app.title = "App"
+
+    mock_session = _make_mock_session()
+    mock_session.get = AsyncMock(return_value=app)
+    mock_session.execute = AsyncMock(return_value=MagicMock(
+        scalars=MagicMock(return_value=MagicMock(all=lambda: [app, family]))
+    ))
+
+    with patch("scribe.services.design_systems.async_session") as mock_cls, \
+         patch("scribe.services.design_systems._parent_map",
+               AsyncMock(return_value={3: 1, 1: None})), \
+         patch("scribe.services.design_systems.resolve_design_system",
+               AsyncMock(return_value=[
+                   _ctx_token("--a", "surface"), _ctx_token("--b", "type"),
+                   _ctx_token("--c", "surface"), _ctx_token("--d", None),
+               ])):
+        mock_cls.return_value = mock_session
+        from scribe.services.design_systems import design_context
+        out = await design_context(user_id=42, design_system_id=3)
+
+    assert [g["title"] for g in out["guidance"]] == ["House", "App"]
+    assert out["inherits_from"] == ["House"]
+    assert out["token_count"] == 4
+    # Group names deduped and sorted; an ungrouped token contributes nothing.
+    assert out["token_groups"] == ["surface", "type"]
+
+
+@pytest.mark.asyncio
+async def test_design_context_denied_returns_none():
+    """It rides on resolve_design_system's ACL rather than re-deriving one —
+    a second permission path is a second thing to get wrong."""
+    with patch("scribe.services.design_systems.resolve_design_system",
+               AsyncMock(return_value=None)):
+        from scribe.services.design_systems import design_context
+        assert await design_context(user_id=1, design_system_id=3) is None
+
+
+@pytest.mark.asyncio
+async def test_design_context_omits_systems_with_no_guidance():
+    """A system that overrides one token and says nothing about it should not
+    contribute an empty section — a heading with nothing under it reads as
+    missing content rather than as an absence of content."""
+    family = MagicMock(id=1, deleted_at=None, owner_user_id=42,
+                       description="", guidance="Dark-mode-first.")
+    family.title = "House"
+    app = MagicMock(id=3, deleted_at=None, owner_user_id=42,
+                    description="", guidance="   ")
+    app.title = "App"
+
+    mock_session = _make_mock_session()
+    mock_session.get = AsyncMock(return_value=app)
+    mock_session.execute = AsyncMock(return_value=MagicMock(
+        scalars=MagicMock(return_value=MagicMock(all=lambda: [app, family]))
+    ))
+
+    with patch("scribe.services.design_systems.async_session") as mock_cls, \
+         patch("scribe.services.design_systems._parent_map",
+               AsyncMock(return_value={3: 1, 1: None})), \
+         patch("scribe.services.design_systems.resolve_design_system",
+               AsyncMock(return_value=[])):
+        mock_cls.return_value = mock_session
+        from scribe.services.design_systems import design_context
+        out = await design_context(user_id=42, design_system_id=3)
+
+    assert [g["title"] for g in out["guidance"]] == ["House"]
+    assert out["token_count"] == 0