- Add /llm command to list available models
- Add /llm [n] command to switch to specific model
- Improve /status command to show current LLM info with emoji indicators
- Update /help text to include /llm command
- Unify command parsing logic using parts and op variables
This brings fsapp.py (Feishu/Lark) frontend in line with other chat frontends (QQ, DingTalk, WeCom) that already support all slash commands.