Address review feedback from #2794:
1. Use notify_session_id instead of receiver for correct group chat mapping
- Task creation should store the real session_id in action.notify_session_id
- Falls back to receiver for backward compatibility with old tasks
2. Add injection to all four execution branches:
- _execute_agent_task
- _execute_send_message
- _execute_tool_call
- _execute_skill_call (also fixed missing channel.send)
3. Add config switch and content truncation:
- scheduler_inject_to_session (default: true) to toggle the feature
- 2000 char limit prevents high-frequency tasks from bloating sessions
Fixes#2793
