feat: Auto-resume idle agents for inter-agent conversations

When an agent asks a question via `cw ask` targeting an idle agent,
the conversation router now auto-resumes the idle agent's session so
it can answer. Previously, questions to idle agents sat unanswered
forever because target resolution only matched running agents.

Changes:
- Add `resumeForConversation()` to AgentManager interface and implement
  on MultiProviderAgentManager (mirrors resumeForCommit pattern)
- Relax createConversation target resolution: prefer running, fall back
  to idle (was running-only)
- Trigger auto-resume after conversation creation for idle targets
- Add concurrency lock (conversationResumeLocks Set) to prevent
  double-resume race conditions

apps/server/dispatch/manager.test.ts

@@ -80,6 +80,7 @@ function createMockAgentManager(
     resume: vi.fn().mockResolvedValue(undefined),
     getResult: vi.fn().mockResolvedValue(null),
     getPendingQuestions: vi.fn().mockResolvedValue(null),
+    resumeForConversation: vi.fn().mockResolvedValue(false),
   };
 }