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/cron.go

@@ -122,10 +122,10 @@ func (t *CronTool) Execute(ctx context.Context, args map[string]interface{}) *To
 }
 
 func (t *CronTool) addJob(args map[string]interface{}) *ToolResult {
-	t.mu.RLock()
+	t.mu.Lock()
 	channel := t.channel
 	chatID := t.chatID
-	t.mu.RUnlock()
+	t.mu.Unlock()
 
 	if channel == "" || chatID == "" {
 		return ErrorResult("no session context (channel/chat_id not set). Use this tool in an active conversation.")