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

web/style.css

@@ -1043,6 +1043,22 @@ body[data-mode="text"] .mic-meter {
     font-size: 0.72rem;
 }
 
+.sheet-section-title {
+    margin: 0.4rem 0 0.55rem;
+    padding: 0 0.2rem;
+    color: var(--ink-muted);
+    font-size: 0.68rem;
+    font-weight: 700;
+    letter-spacing: 0.14em;
+    text-transform: uppercase;
+}
+
+.sheet-note {
+    margin: 0.5rem 0 0;
+    color: var(--ink-muted);
+    font-size: 0.72rem;
+}
+
 .voice-list {
     min-height: 0;
     overflow-y: auto;
@@ -1052,6 +1068,11 @@ body[data-mode="text"] .mic-meter {
     padding: 0.15rem 0.2rem 0 0;
 }
 
+/* Two pickers (voice + brain) share the settings sheet height. */
+.settings-sheet .voice-list {
+    flex: 1 1 0;
+}
+
 .voice-card {
     min-height: 54px;
     display: flex;