feat(CF-1189): Add AI text bot + WildFiles RAG integration

Extends bot.py with text message handling:
- RoomMessageText callback with @mention detection
- LLM responses via LiteLLM (OpenAI-compatible)
- WildFiles document search (DocumentRAG class)
- Per-room model selection via room state events
- Commands: !ai help/models/set-model/search
- Typing indicators during AI response generation
- 30s staleness check to avoid replaying history

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

requirements.txt

@@ -6,3 +6,5 @@ livekit>=1.0,<2.0
 livekit-api>=1.0,<2.0
 matrix-nio[e2e]>=0.25,<1.0
 canonicaljson>=2.0,<3.0
+httpx>=0.27,<1.0
+openai>=1.0,<2.0