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>

frontend/src/api/rulebooks.ts

@@ -22,7 +22,8 @@ export interface RulebookTopic {
 
 export interface Rule {
   id: number;
-  topic_id: number;
+  topic_id: number | null;
+  project_id: number | null;
   title: string;
   statement: string;
   why: string;
@@ -36,7 +37,7 @@ export interface RuleHeader {
   id: number;
   title: string;
   statement: string;
-  topic_id: number;
+  topic_id: number | null;
 }
 
 export interface ApplicableRules {
@@ -47,6 +48,11 @@ export interface ApplicableRules {
     topic_title: string;
     rulebook_title: string;
   }[];
+  project_rules: {
+    id: number;
+    title: string;
+    statement: string;
+  }[];
   truncated: boolean;
   subscribed_rulebooks: { id: number; title: string }[];
 }
@@ -134,3 +140,10 @@ export async function unsubscribeProject(projectId: number, rulebookId: number):
 export async function getProjectApplicableRules(projectId: number): Promise<ApplicableRules> {
   return apiGet(`/api/projects/${projectId}/rules`);
 }
+
+export async function createProjectRule(
+  projectId: number,
+  data: { statement: string; title?: string; why?: string; how_to_apply?: string },
+): Promise<Rule> {
+  return apiPost(`/api/projects/${projectId}/rules`, data);
+}