feat: add DMG build script and enhance README with installation instructions and Start at Login feature

apps/mac/AIWorkspace/README.md

@@ -28,6 +28,20 @@ Install to `~/Applications/AIWorkspace.app`:
 apps/mac/AIWorkspace/scripts/package-app.sh --install
 ```
 
+## Build a DMG
+
+```bash
+apps/mac/AIWorkspace/scripts/build-dmg.sh
+```
+
+This creates:
+
+```text
+apps/mac/AIWorkspace/dist/AIWorkspace.dmg
+```
+
+The DMG contains `AIWorkspace.app` and an `Applications` shortcut for drag-and-drop installation.
+
 One-step local install, optionally enabling start at login and opening the app:
 
 ```bash
@@ -36,7 +50,9 @@ apps/mac/AIWorkspace/scripts/install.sh --start-at-login --open
 
 ## Start at login
 
-After installing the app bundle:
+Preferred: open the installed app and use the **Start at Login** toggle in the UI. The app uses macOS `SMAppService` for login item registration.
+
+Development fallback after installing the app bundle:
 
 ```bash
 apps/mac/AIWorkspace/scripts/install-start-at-login.sh
@@ -74,4 +90,5 @@ swift run --package-path apps/mac/AIWorkspace AIWorkspace
 
 - This is not yet packaged as a signed `.app` bundle.
 - Start at login should be implemented later through a LaunchAgent or app login item.
+- Start at Login is available in the app UI through `SMAppService`. The LaunchAgent scripts are retained as development fallback utilities.
 - The app should remain a UI layer; service lifecycle remains in `scripts/aiw/services.py`.