feat: enrich listWaitingAgents with task/phase/initiative context via DB joins

Replaces the in-memory filter (agentManager.list() + filter) with a direct
repository query that LEFT JOINs tasks, phases, and initiatives to return
taskName, phaseName, initiativeName, and taskDescription alongside agent fields.

- Adds AgentWithContext interface and findWaitingWithContext() to AgentRepository port
- Implements findWaitingWithContext() in DrizzleAgentRepository using getTableColumns
- Wires agentRepository into TRPCContext, CreateContextOptions, and TrpcAdapterOptions
- Adds requireAgentRepository() helper following existing pattern
- Updates listWaitingAgents to use repository query instead of agentManager
- Adds 5 unit tests for findWaitingWithContext() covering all FK join edge cases
- Updates existing AgentRepository mocks to satisfy updated interface

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/server/agent/mutex-completion.test.ts

@@ -46,7 +46,8 @@ describe('OutputHandler completion mutex', () => {
     async findByTaskId() { throw new Error('Not implemented'); },
     async findByName() { throw new Error('Not implemented'); },
     async findBySessionId() { throw new Error('Not implemented'); },
-    async delete() { throw new Error('Not implemented'); }
+    async delete() { throw new Error('Not implemented'); },
+    async findWaitingWithContext() { throw new Error('Not implemented'); }
   };
 
   beforeEach(() => {