feat(coding-agent): add /quit and /exit slash commands to exit (#426)

Add new slash commands for gracefully exiting the interactive mode:
- /quit - exit the application
- /exit - alias for /quit (common CLI convention)

Both commands await shutdown() which emits session_shutdown events
to hooks and custom tools before exiting. Unlike Ctrl+C (twice) which
uses void (fire-and-forget), these commands properly wait for hooks
and tools to complete their shutdown handlers.

packages/coding-agent/src/modes/interactive/interactive-mode.ts

@@ -891,6 +891,11 @@ export class InteractiveMode {
 				this.editor.setText("");
 				return;
 			}
+			if (text === "/quit" || text === "/exit") {
+				this.editor.setText("");
+				await this.shutdown();
+				return;
+			}
 
 			// Handle bash command (! for normal, !! for excluded from context)
 			if (text.startsWith("!")) {