feat: switch LLM to OpenWebUI with live model selection

- Point the agent at OpenWebUI's OpenAI-compatible API (LLM_BASE_URL,
  LLM_API_KEY, LLM_MODEL) with GEMMA_* env fallbacks
- Add list_available_models() fetching {base_url}/models, filtering
  non-chat ids (embedding/reranker)
- Rename GemmaLLM -> HopeLLM with update_options(model=...) for runtime
  model switching (mutates _opts.model) and instance-based compaction
- Data channel: agent publishes {type: models} after connect and on
  get_models; set_model switches live and acks model_changed
- Web UI: Brain section in the settings sheet with a model picker that
  persists the choice in localStorage
- Update tests, CI workflow env, and docs for the new endpoint

.env.example

@@ -5,10 +5,11 @@ AZURE_SPEECH_REGION=eastus
 # Default TTS voice (Azure Dragon HD Omni; supports inline delivery markers)
 AZURE_TTS_VOICE=en-US-Ava:DragonHDOmniLatestNeural
 
-# LLM (Gemma on xNAS via llama.cpp, OpenAI-compatible)
-GEMMA_BASE_URL=http://192.168.86.2:8023/v1
-GEMMA_MODEL=gemma-4-e4b
-GEMMA_API_KEY=not-needed
+# LLM (OpenWebUI on xNAS, OpenAI-compatible; also serves the local llama.cpp
+# models. The model can be switched live from the web UI settings sheet.)
+LLM_BASE_URL=http://192.168.86.2:3005/api
+LLM_MODEL=gemma-4-e4b
+LLM_API_KEY=sk-your-openwebui-api-key
 
 # LiveKit (generated in livekit.yaml, referenced here for the agent)
 LIVEKIT_URL=http://localhost:7880