feat: support "Default (CLI default)" option for model selection

Allow users to leave the default model unset, so sessions use whatever
the Claude CLI defaults to rather than forcing a specific model.

- Add empty-value "Default (CLI default)" option to the model dropdown
- Treat empty string as undefined when passing model to Session
- Apply consistently across session creation, quick-start, and Ralph

src/web/public/index.html

@@ -1085,6 +1085,7 @@
             <div class="form-row">
               <label>Default Model</label>
               <select id="appSettingsDefaultModel" class="form-select">
+                <option value="">Default (CLI default)</option>
                 <option value="opus">Opus (Most capable)</option>
                 <option value="sonnet">Sonnet (Balanced)</option>
                 <option value="haiku">Haiku (Fast & cheap)</option>

src/web/public/settings-ui.js

@@ -1278,7 +1278,7 @@ Object.assign(CodemanApp.prototype, {
         // Default model
         const defaultModelEl = document.getElementById('appSettingsDefaultModel');
         if (defaultModelEl) {
-          defaultModelEl.value = config.defaultModel || 'opus';
+          defaultModelEl.value = config.defaultModel || '';
         }
         // Show recommendations
         const showRecsEl = document.getElementById('appSettingsShowModelRecommendations');
@@ -1317,7 +1317,7 @@ Object.assign(CodemanApp.prototype, {
     if (reviewEl?.value) agentTypeOverrides.review = reviewEl.value;
 
     const config = {
-      defaultModel: defaultModelEl?.value || 'opus',
+      defaultModel: defaultModelEl?.value || '',
       showRecommendations: showRecsEl?.checked ?? true,
       agentTypeOverrides,
     };