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

tests/test_llm_api.py

@@ -1,10 +1,10 @@
-"""Tests for the Gemma LLM API — the exact endpoint Hope's agent uses."""
+"""Tests for the LLM API — the exact endpoint Hope's agent uses."""
 
 import time
 
 import pytest
 
-from conftest import GEMMA_BASE_URL, GEMMA_MODEL
+from conftest import LLM_BASE_URL, LLM_MODEL
 
 WEATHER_TOOL = {
     "type": "function",
@@ -22,7 +22,7 @@ WEATHER_TOOL = {
 
 def _chat(llm_client, messages, *, tools=None, stream=False, max_tokens=200):
     payload = {
-        "model": GEMMA_MODEL,
+        "model": LLM_MODEL,
         "messages": messages,
         "max_tokens": max_tokens,
         "temperature": 0.3,