fix: resolve code review issues in tool-result-refactor

1. Remove duplicate ToolResult definition in heartbeat package
   - Import tools.ToolResult instead of local definition
   - Add nil check for handler before execution

2. Fix SpawnTool to return AsyncResult and implement AsyncTool
   - Add callback field and SetCallback method
   - Return AsyncResult instead of NewToolResult

3. Add context cancellation support to SubagentManager
   - Check ctx.Done() before and during task execution
   - Set task status to "cancelled" on cancellation
   - Call callback with result on completion

4. Fix data race window in CronTool.addJob
   - Use Lock instead of RLock for channel/chatID access
   - Ensure consistent snapshot during job creation

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

pkg/tools/spawn.go

@@ -9,6 +9,7 @@ type SpawnTool struct {
 	manager       *SubagentManager
 	originChannel string
 	originChatID  string
+	callback      AsyncCallback // For async completion notification
 }
 
 func NewSpawnTool(manager *SubagentManager) *SpawnTool {
@@ -19,6 +20,11 @@ func NewSpawnTool(manager *SubagentManager) *SpawnTool {
 	}
 }
 
+// SetCallback implements AsyncTool interface for async completion notification
+func (t *SpawnTool) SetCallback(cb AsyncCallback) {
+	t.callback = cb
+}
+
 func (t *SpawnTool) Name() string {
 	return "spawn"
 }
@@ -61,10 +67,12 @@ func (t *SpawnTool) Execute(ctx context.Context, args map[string]interface{}) *T
 		return ErrorResult("Subagent manager not configured")
 	}
 
-	result, err := t.manager.Spawn(ctx, task, label, t.originChannel, t.originChatID)
+	// Pass callback to manager for async completion notification
+	result, err := t.manager.Spawn(ctx, task, label, t.originChannel, t.originChatID, t.callback)
 	if err != nil {
 		return ErrorResult(fmt.Sprintf("failed to spawn subagent: %v", err))
 	}
 
-	return NewToolResult(result)
+	// Return AsyncResult since the task runs in background
+	return AsyncResult(result)
 }