feat: Add errand tRPC router with all 9 procedures and comprehensive tests

Implements the errand workflow for small isolated changes that spawn a
dedicated agent in a git worktree:
- errand.create: branch + worktree + DB record + agent spawn
- errand.list / errand.get / errand.diff: read procedures
- errand.complete: transitions active→pending_review, stops agent
- errand.merge: merges branch, handles conflicts with conflictFiles
- errand.delete / errand.abandon: cleanup worktree, branch, agent
- errand.sendMessage: delivers user message directly to running agent

Supporting changes:
- Add 'errand' to AgentMode union and agents.mode enum
- Add sendUserMessage() to AgentManager interface and MockAgentManager
- MockAgentManager now accepts optional agentRepository to persist agents
  to the DB (required for FK constraint satisfaction in tests)
- Add ORDER BY createdAt DESC, id DESC to errand findAll
- Fix dispatch/manager.test.ts missing sendUserMessage mock

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

apps/server/dispatch/manager.test.ts

@@ -81,6 +81,7 @@ function createMockAgentManager(
     getResult: vi.fn().mockResolvedValue(null),
     getPendingQuestions: vi.fn().mockResolvedValue(null),
     resumeForConversation: vi.fn().mockResolvedValue(false),
+    sendUserMessage: vi.fn().mockResolvedValue(undefined),
   };
 }