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/mcp/server.py

@@ -47,13 +47,14 @@ project subscribes to) and subscribed_rulebooks; consult those too. Full text
 (Why / How-to-apply) is available via get_rule(id).
 
 Engineering and workflow rules live in Scribe. When you notice a pattern
-worth codifying, call create_rule. Do NOT add new engineering rules to
-CLAUDE.md or to ~/.claude/.../memory/feedback_*.md — those stores are
-reserved for facts about the user (preferences, role, communication style)
-and codebase onboarding pointers, respectively. Before creating a rule,
-call list_always_on_rules and list_rules(project_id=...) to avoid duplicates.
-Coordinate with the operator on whether a new rule belongs in an existing
-rulebook+topic or a new one.
+worth codifying, call create_rule (cross-project, lands in a rulebook+topic)
+or create_project_rule (one project only, no rulebook ceremony). Do NOT add
+new engineering rules to CLAUDE.md or to ~/.claude/.../memory/feedback_*.md
+— those stores are reserved for facts about the user (preferences, role,
+communication style) and codebase onboarding pointers, respectively. Before
+creating a rule, call list_always_on_rules and list_rules(project_id=...) to
+avoid duplicates. Coordinate with the operator on whether a new rule belongs
+in a project, an existing rulebook+topic, or a new rulebook.
 
 Plans are tasks with kind=plan, and Scribe is the canonical home for them.
 When you begin non-trivial work, call start_planning(project_id, title) FIRST —