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>

src/fabledassistant/models/rulebook.py

@@ -81,8 +81,18 @@ class Rule(Base, SoftDeleteMixin):
     )
 
     id: Mapped[int] = mapped_column(BigInteger, primary_key=True)
-    topic_id: Mapped[int] = mapped_column(
-        BigInteger, ForeignKey("rulebook_topics.id", ondelete="CASCADE")
+    # Exactly one of topic_id / project_id is set — enforced by CHECK
+    # constraint ck_rule_topic_xor_project (migration 0059).
+    topic_id: Mapped[int | None] = mapped_column(
+        BigInteger,
+        ForeignKey("rulebook_topics.id", ondelete="CASCADE"),
+        nullable=True,
+    )
+    project_id: Mapped[int | None] = mapped_column(
+        BigInteger,
+        ForeignKey("projects.id", ondelete="CASCADE"),
+        nullable=True,
+        index=True,
     )
     title: Mapped[str] = mapped_column(Text)
     statement: Mapped[str] = mapped_column(Text)
@@ -102,6 +112,7 @@ class Rule(Base, SoftDeleteMixin):
         return {
             "id": self.id,
             "topic_id": self.topic_id,
+            "project_id": self.project_id,
             "title": self.title,
             "statement": self.statement,
             "why": self.why or "",