feat(rules): project-scoped rules (S3)

Rules can now belong to either a rulebook topic OR a single project,
enforced by a CHECK constraint (exactly-one of topic_id/project_id).
Adds the create_project_rule MCP tool + REST endpoint, surfaces
project-scoped rules in get_project/get_task/start_planning under a
new project_rules field, and adds a project Rules tab section with an
inline create form so the operator can author project rules from the
UI without rulebook ceremony.

- migration 0059: rules.project_id (FK projects ON DELETE CASCADE),
  topic_id now nullable, CHECK ck_rule_topic_xor_project, index on
  project_id
- model: Rule gains project_id; to_dict exposes it
- service: create_project_rule with project-ownership guard; list_rules
  with project_id filter UNIONs subscription-derived + project-scoped;
  get_applicable_rules adds a project_rules field; get_rule / update_rule
  / delete_rule fetch via a shared _fetch_owned_rule that handles both
  rulebook and project ownership paths
- trash: project delete cascades to project-scoped rules
- MCP: create_project_rule tool registered; _INSTRUCTIONS mentions both
  create_rule and create_project_rule paths
- REST: POST /api/projects/<id>/rules (statement required, title derived
  if omitted)
- frontend: Rule type gains nullable topic_id + project_id; createProjectRule
  client; ProjectRulesTab.vue gains a "Project rules" section with inline
  create form and per-rule expand/delete
- tests: register count → 18; create_project_rule unit tests (required
  fields, title derivation, explicit-title pass-through); applicable_rules
  shape tests now include project_rules; trash cascade test updated to
  expect 5 executions

S1+S2 (always_on flag + Scribe-first prompt) shipped in 658348f.
S4 (enter_project handshake) follows.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/test_mcp_tool_rulebooks.py

@@ -179,12 +179,13 @@ def test_register_attaches_all_sixteen_tools():
             return decorator
 
     register(FakeMCP())
-    assert len(registered) == 17
+    assert len(registered) == 18
     # spot-check a few names
     assert "list_rulebooks" in registered
     assert "create_rule" in registered
     assert "subscribe_project_to_rulebook" in registered
     assert "list_always_on_rules" in registered
+    assert "create_project_rule" in registered
 
 
 @pytest.mark.asyncio
@@ -235,3 +236,51 @@ async def test_update_rulebook_omits_always_on_when_none():
     kwargs = mock.call_args.kwargs
     assert "always_on" not in kwargs
     assert kwargs["title"] == "new title"
+
+
+@pytest.mark.asyncio
+async def test_create_project_rule_passes_required_fields():
+    rule = _fake_rule()
+    mock = AsyncMock(return_value=rule)
+    with patch("fabledassistant.mcp.tools.rulebooks.rulebooks_svc.create_project_rule", mock):
+        from fabledassistant.mcp.tools.rulebooks import create_project_rule
+        await create_project_rule(
+            project_id=42,
+            statement="Always run migrations through alembic, not raw SQL.",
+            why="audit trail",
+        )
+    kwargs = mock.call_args.kwargs
+    assert kwargs["user_id"] == 7
+    assert kwargs["project_id"] == 42
+    assert kwargs["statement"].startswith("Always run migrations")
+    assert kwargs["why"] == "audit trail"
+
+
+@pytest.mark.asyncio
+async def test_create_project_rule_derives_title_from_statement():
+    rule = _fake_rule()
+    mock = AsyncMock(return_value=rule)
+    with patch("fabledassistant.mcp.tools.rulebooks.rulebooks_svc.create_project_rule", mock):
+        from fabledassistant.mcp.tools.rulebooks import create_project_rule
+        await create_project_rule(
+            project_id=42,
+            statement="Avoid auto-generated docstrings. Reviewers find them noise.",
+        )
+    kwargs = mock.call_args.kwargs
+    # Title should be derived from the first sentence, capped at 50 chars
+    assert kwargs["title"] == "Avoid auto-generated docstrings"
+
+
+@pytest.mark.asyncio
+async def test_create_project_rule_uses_explicit_title_when_given():
+    rule = _fake_rule()
+    mock = AsyncMock(return_value=rule)
+    with patch("fabledassistant.mcp.tools.rulebooks.rulebooks_svc.create_project_rule", mock):
+        from fabledassistant.mcp.tools.rulebooks import create_project_rule
+        await create_project_rule(
+            project_id=42,
+            statement="anything",
+            title="no auto-docstrings",
+        )
+    kwargs = mock.call_args.kwargs
+    assert kwargs["title"] == "no auto-docstrings"