fix: default agent profile to 'Auto (Optimized)' for all users

Add one-time migration to reset selectedAgentProfile to 'auto' for
existing users. This ensures the optimized per-phase model selection
is the default experience for everyone.

The migration:
- Runs once on settings load (tracked by _migratedAgentProfileToAuto flag)
- Sets selectedAgentProfile to 'auto'
- Persists the change to settings.json
- Users can still change their preference afterward

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

auto-claude-ui/src/main/ipc-handlers/settings-handlers.ts

@@ -95,6 +95,7 @@ export function registerSettingsHandlers(
     IPC_CHANNELS.SETTINGS_GET,
     async (): Promise<IPCResult<AppSettings>> => {
       let settings = { ...DEFAULT_APP_SETTINGS };
+      let needsSave = false;
 
       if (existsSync(settingsPath)) {
         try {
@@ -105,6 +106,15 @@ export function registerSettingsHandlers(
         }
       }
 
+      // Migration: Reset agent profile to 'auto' for existing users (one-time)
+      // This ensures all users get the optimized 'auto' profile as the default
+      const settingsAny = settings as Record<string, unknown>;
+      if (!settingsAny._migratedAgentProfileToAuto) {
+        settings.selectedAgentProfile = 'auto';
+        settingsAny._migratedAgentProfileToAuto = true;
+        needsSave = true;
+      }
+
       // If no manual autoBuildPath is set, try to auto-detect
       if (!settings.autoBuildPath) {
         const detectedPath = detectAutoBuildSourcePath();
@@ -113,6 +123,15 @@ export function registerSettingsHandlers(
         }
       }
 
+      // Persist migration changes
+      if (needsSave) {
+        try {
+          writeFileSync(settingsPath, JSON.stringify(settings, null, 2));
+        } catch {
+          // Ignore write errors during migration
+        }
+      }
+
       return { success: true, data: settings as AppSettings };
     }
   );