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/routes/rulebooks.py

@@ -239,3 +239,27 @@ async def get_project_rules(project_id: int):
         project_id=project_id, user_id=_uid(),
     )
     return jsonify(result)
+
+
+@rulebooks_bp.post("/projects/<int:project_id>/rules")
+@login_required
+async def create_project_rule(project_id: int):
+    """Create a rule scoped to a single project. Frontend fast path."""
+    data = await request.get_json() or {}
+    statement = (data.get("statement") or "").strip()
+    if not statement:
+        return jsonify({"error": "statement is required"}), 400
+    title = (data.get("title") or "").strip() or statement.split(".")[0][:50]
+    try:
+        rule = await rulebooks_svc.create_project_rule(
+            project_id=project_id,
+            user_id=_uid(),
+            title=title,
+            statement=statement,
+            why=data.get("why", ""),
+            how_to_apply=data.get("how_to_apply", ""),
+            order_index=data.get("order_index", 0),
+        )
+    except ValueError as exc:
+        return jsonify({"error": str(exc)}), 404
+    return jsonify(rule.to_dict()), 201