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/index.html

@@ -75,7 +75,7 @@
                     <button id="muteBtn" class="btn btn-icon" aria-label="Mute microphone" title="Mute microphone" disabled>
                         <svg viewBox="0 0 24 24" width="19" height="19" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M12 2a3 3 0 0 0-3 3v7a3 3 0 0 0 6 0V5a3 3 0 0 0-3-3Z"></path><path d="M19 10v2a7 7 0 0 1-14 0v-2"></path><line x1="8" y1="22" x2="16" y2="22"></line></svg>
                     </button>
-                    <button id="settingsBtn" class="btn btn-icon" aria-label="Voice settings" title="Voice settings">
+                    <button id="settingsBtn" class="btn btn-icon" aria-label="Voice and brain settings" title="Voice and brain settings">
                         <svg viewBox="0 0 24 24" width="19" height="19" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M4 7h10M18 7h2M4 17h2M10 17h10"></path><circle cx="16" cy="7" r="2"></circle><circle cx="8" cy="17" r="2"></circle></svg>
                     </button>
                 </div>
@@ -113,18 +113,22 @@
     </main>
 
     <div class="sheet-backdrop" id="sheetBackdrop" hidden></div>
-    <div class="settings-sheet" id="settingsSheet" role="dialog" aria-modal="true" aria-label="Voice settings">
+    <div class="settings-sheet" id="settingsSheet" role="dialog" aria-modal="true" aria-label="Hope settings">
         <div class="sheet-handle" aria-hidden="true"></div>
         <div class="sheet-header">
             <div>
-                <h2>Choose Hope’s voice</h2>
-                <p>Changes apply to this conversation.</p>
+                <h2>Hope’s settings</h2>
+                <p>Voice and brain — changes apply to this conversation.</p>
             </div>
-            <button id="closeSheetBtn" class="btn btn-icon" aria-label="Close voice settings">
+            <button id="closeSheetBtn" class="btn btn-icon" aria-label="Close settings">
                 <svg viewBox="0 0 24 24" width="18" height="18" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" aria-hidden="true"><path d="M18 6 6 18M6 6l12 12"></path></svg>
             </button>
         </div>
+        <h3 class="sheet-section-title">Voice</h3>
         <div class="voice-list" id="voiceList"></div>
+        <h3 class="sheet-section-title">Brain</h3>
+        <div class="voice-list" id="modelList"></div>
+        <p class="sheet-note" id="modelNote" hidden></p>
     </div>
 
     <div class="tasks-panel" id="tasksPanel" aria-label="Background tasks">