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,
     };

src/web/routes/ralph-routes.ts

@@ -308,7 +308,7 @@ export function registerRalphRoutes(
       useMux: true,
       mode: 'claude',
       niceConfig,
-      model: rlModelConfig?.defaultModel,
+      model: rlModelConfig?.defaultModel || undefined,
       claudeMode: rlClaudeModeConfig.claudeMode,
       allowedTools: rlClaudeModeConfig.allowedTools,
     });

src/web/routes/session-routes.ts

@@ -186,7 +186,11 @@ export function registerSessionRoutes(
     const modelConfig = await ctx.getModelConfig();
     const mode = body.mode || 'claude';
     const model =
-      mode === 'opencode' ? body.openCodeConfig?.model : mode !== 'shell' ? modelConfig?.defaultModel : undefined;
+      mode === 'opencode'
+        ? body.openCodeConfig?.model
+        : mode !== 'shell'
+          ? modelConfig?.defaultModel || undefined
+          : undefined;
     const claudeModeConfig = await ctx.getClaudeModeConfig();
     const session = new Session({
       workingDir,
@@ -766,7 +770,11 @@ export function registerSessionRoutes(
     const niceConfig = await ctx.getGlobalNiceConfig();
     const qsModelConfig = await ctx.getModelConfig();
     const qsModel =
-      mode === 'opencode' ? openCodeConfig?.model : mode !== 'shell' ? qsModelConfig?.defaultModel : undefined;
+      mode === 'opencode'
+        ? openCodeConfig?.model
+        : mode !== 'shell'
+          ? qsModelConfig?.defaultModel || undefined
+          : undefined;
     const qsClaudeModeConfig = await ctx.getClaudeModeConfig();
     const session = new Session({
       workingDir: casePath,