feat(capability-requests): add cross-domain capability catalog and request routing

Introduces a capability catalog (CUST-WP-0022) so domains can advertise what
they provide and agents can request capabilities from other domains with
auto-routing, lifecycle tracking, and task-unblocking on completion.

- New models: CapabilityCatalog, CapabilityRequest with full lifecycle
  (requested → accepted → in_progress → ready_for_review → completed/rejected/withdrawn)
- Migration i6d7e8f9a0b1: capability_catalog + capability_requests tables
- Router /capability-catalog and /capability-requests with accept/status endpoints
- 7 new MCP tools: register_capability, list_capabilities, request_capability,
  accept_capability_request, update_capability_request_status,
  list_capability_requests, get_capability_request
- StateSummary gains open_capability_requests count
- Dashboard: capability-requests.md page + docs/capabilities.md + docs/scope.md
- SCOPE.md: three seed capabilities documented (MCP registration, state tracking, SBOM)
- scope.template: Provided Capabilities section with example block
- scripts/ingest_capabilities.py + make ingest-capabilities[/-all] targets

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

api/main.py

@@ -6,7 +6,7 @@ from fastapi.middleware.cors import CORSMiddleware
 
 from api.database import engine
 from api.routers import decisions, extension_points, progress, state, tasks, technical_debt, topics, workstreams, workstream_dependencies
-from api.routers import domains, repos, contributions, sbom, policy, domain_goals, repo_goals, messages
+from api.routers import domains, repos, contributions, sbom, policy, domain_goals, repo_goals, messages, capability_requests
 
 
 @asynccontextmanager
@@ -47,6 +47,7 @@ app.include_router(repo_goals.router)
 app.include_router(contributions.router)
 app.include_router(sbom.router)
 app.include_router(messages.router)
+app.include_router(capability_requests.router)
 app.include_router(state.router)
 app.include_router(policy.router)