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

supervisord.conf

@@ -27,9 +27,9 @@ environment=
     AZURE_SPEECH_KEY="%(ENV_AZURE_SPEECH_KEY)s",
     AZURE_SPEECH_REGION="%(ENV_AZURE_SPEECH_REGION)s",
     AZURE_TTS_VOICE="%(ENV_AZURE_TTS_VOICE)s",
-    GEMMA_BASE_URL="%(ENV_GEMMA_BASE_URL)s",
-    GEMMA_MODEL="%(ENV_GEMMA_MODEL)s",
-    GEMMA_API_KEY="%(ENV_GEMMA_API_KEY)s",
+    LLM_BASE_URL="%(ENV_LLM_BASE_URL)s",
+    LLM_MODEL="%(ENV_LLM_MODEL)s",
+    LLM_API_KEY="%(ENV_LLM_API_KEY)s",
     WEB_MCP_ENABLED="%(ENV_WEB_MCP_ENABLED)s",
     FIRECRAWL_BASE="%(ENV_FIRECRAWL_BASE)s",
     VOICE_USERS_FILE="%(ENV_VOICE_USERS_FILE)s"