feat(011-agent-ticket-queue): link agents to accounts, list assigned tickets

Extends PATCH /admin/agents/:agentId with an optional userId to finally
wire Agent.userId (added in 010-identity-auth as schema-only, never
consumed by any workflow), with proactive role/duplicate-link checks
mirroring UsersService.create's own pre-check style.

Adds GET /agents/me/tickets and GET /admin/agents/:agentId/tickets,
sharing one TicketsService.listAssignedTo method, returning a dashboard-
ready summary (product, customer, priority, severity, status, SLA state)
of every ticket currently assigned to an agent — no such query existed
anywhere in the ticketing or orchestration modules before this. Backed by
a new Assignment @@index([agentId, isCurrent]).

Discovered while starting supporthub-web's 001-agent-admin-ui: its agent-
dashboard user story had no backend data source without this.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

specs/011-agent-ticket-queue/checklists/requirements.md

@@ -46,3 +46,13 @@
   the one query supporthub-web's agent dashboard actually needs, to avoid speculative scope
   beyond what 001-agent-admin-ui's own spec calls for.
 - All items pass; no revision iterations were needed.
+- **Implementation-time finding**: research.md's plan to add a dedicated
+  `AgentsService.requireAgentForUser` guard (rather than inlining the lookup in the ticketing
+  controller) turned out to matter for testability, not just style — it let T007's unit test
+  exercise the "no linked agent" rejection with a fake repository, with no real database
+  involved, exactly the kind of isolated unit coverage tasks.md asked for. Worth defaulting to
+  this shape (a small service method over inline controller logic) whenever a cross-module
+  guard needs its own unit test.
+- No other deviations from plan.md — the two-routes-sharing-one-service-method design, the
+  proactive existence/role/duplicate-link checks, and the new composite index all worked exactly
+  as researched, and the full regression suite (unit + integration) stayed clean throughout.