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/test/harness.ts

@@ -254,6 +254,11 @@ export interface TestHarness {
    */
   getPendingQuestions(agentId: string): Promise<PendingQuestions | null>;
 
+  /**
+   * Resume an idle agent to answer a conversation (mock: always returns false).
+   */
+  resumeForConversation(agentId: string, conversationId: string, question: string, fromAgentId: string): Promise<boolean>;
+
   /**
    * Get events by type.
    */
@@ -505,6 +510,9 @@ export function createTestHarness(): TestHarness {
 
     getPendingQuestions: (agentId: string) => agentManager.getPendingQuestions(agentId),
 
+    resumeForConversation: (agentId: string, conversationId: string, question: string, fromAgentId: string) =>
+      agentManager.resumeForConversation(agentId, conversationId, question, fromAgentId),
+
     getEventsByType: (type: string) => eventBus.getEventsByType(type),
 
     getEmittedEvents: (type: string) => eventBus.getEventsByType(type),