feat(06-02): enhance conflict bounce-back with agent messaging

Adds MessageRepository to DefaultCoordinationManager for conflict notification:
- Creates info message to agent when merge conflict occurs
- Message includes conflicting file list and resolution instructions
- System-generated messages use senderType='user' with null senderId
- Refactors conflict description into cleaner array.join format

src/coordination/manager.ts

@@ -18,6 +18,7 @@ import type {
 import type { WorktreeManager } from '../git/types.js';
 import type { TaskRepository } from '../db/repositories/task-repository.js';
 import type { AgentRepository } from '../db/repositories/agent-repository.js';
+import type { MessageRepository } from '../db/repositories/message-repository.js';
 import type { CoordinationManager, MergeQueueItem, MergeResult } from './types.js';
 
 // =============================================================================
@@ -55,6 +56,7 @@ export class DefaultCoordinationManager implements CoordinationManager {
     private worktreeManager?: WorktreeManager,
     private taskRepository?: TaskRepository,
     private agentRepository?: AgentRepository,
+    private messageRepository?: MessageRepository,
     private eventBus?: EventBus
   ) {}
 
@@ -257,24 +259,45 @@ export class DefaultCoordinationManager implements CoordinationManager {
 
   /**
    * Handle a merge conflict.
-   * Creates a conflict-resolution task and assigns back to the agent.
+   * Creates a conflict-resolution task and notifies the agent via message.
    */
   async handleConflict(taskId: string, conflicts: string[]): Promise<void> {
     if (!this.taskRepository) {
       throw new Error('TaskRepository not configured');
     }
 
+    if (!this.agentRepository) {
+      throw new Error('AgentRepository not configured');
+    }
+
     // Get original task for context
     const originalTask = await this.taskRepository.findById(taskId);
     if (!originalTask) {
       throw new Error(`Original task not found: ${taskId}`);
     }
 
+    // Get agent that was working on the task
+    const agent = await this.agentRepository.findByTaskId(taskId);
+    if (!agent) {
+      throw new Error(`No agent found for task: ${taskId}`);
+    }
+
+    // Build conflict description
+    const conflictDescription = [
+      'Merge conflicts detected. Resolve conflicts in the following files:',
+      '',
+      ...conflicts.map((f) => `- ${f}`),
+      '',
+      `Original task: ${originalTask.name}`,
+      '',
+      'Instructions: Resolve merge conflicts in the listed files, then mark task complete.',
+    ].join('\n');
+
     // Create new conflict-resolution task
     const conflictTask = await this.taskRepository.create({
       planId: originalTask.planId,
       name: `Resolve conflicts: ${originalTask.name}`,
-      description: `Merge conflicts detected. Resolve conflicts in the following files:\n\n${conflicts.map((f) => `- ${f}`).join('\n')}\n\nOriginal task: ${originalTask.name}\n\nInstructions: Resolve merge conflicts in the listed files, then mark task complete.`,
+      description: conflictDescription,
       type: 'auto',
       priority: 'high', // Conflicts should be resolved quickly
       status: 'pending',
@@ -284,6 +307,30 @@ export class DefaultCoordinationManager implements CoordinationManager {
     // Update original task status to blocked
     await this.taskRepository.update(taskId, { status: 'blocked' });
 
+    // Create message to agent if messageRepository is configured
+    if (this.messageRepository) {
+      const messageContent = [
+        `Merge conflict detected for task: ${originalTask.name}`,
+        '',
+        'Conflicting files:',
+        ...conflicts.map((f) => `- ${f}`),
+        '',
+        `A new task has been created to resolve these conflicts: ${conflictTask.name}`,
+        '',
+        'Please resolve the merge conflicts in the listed files and mark the resolution task as complete.',
+      ].join('\n');
+
+      await this.messageRepository.create({
+        senderType: 'user', // System-generated messages appear as from user
+        senderId: null,
+        recipientType: 'agent',
+        recipientId: agent.id,
+        type: 'info',
+        content: messageContent,
+        requiresResponse: false,
+      });
+    }
+
     // Emit TaskQueuedEvent for the new conflict-resolution task
     const event: TaskQueuedEvent = {
       type: 'task:queued',