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

.gitea/workflows/rebuild.yml

@@ -40,9 +40,9 @@ jobs:
             -e AZURE_SPEECH_KEY="$AZURE_SPEECH_KEY" \
             -e AZURE_SPEECH_REGION=eastus \
             -e AZURE_TTS_VOICE=en-US-Ava:DragonHDOmniLatestNeural \
-            -e GEMMA_BASE_URL=http://192.168.86.2:8023/v1 \
-            -e GEMMA_MODEL=gemma-4-e4b \
-            -e GEMMA_API_KEY=not-needed \
+            -e LLM_BASE_URL=http://192.168.86.2:3005/api \
+            -e LLM_MODEL=gemma-4-e4b \
+            -e LLM_API_KEY=sk-4e9259e3bd56472db57fd40adf0fe567 \
             -e WEB_MCP_ENABLED=true \
             -e FIRECRAWL_BASE=http://192.168.86.2:3002 \
             -e WEB_PORT=9178 \