feat(coding-agent): add timeout option to extension dialogs with live countdown

Extension UI dialogs (select, confirm, input) now support a timeout option
that auto-dismisses with a live countdown display. Simpler alternative to
manually managing AbortSignal for timed dialogs.

Also adds ExtensionUIDialogOptions type export and updates RPC mode to
forward timeout to clients.

packages/coding-agent/src/index.ts

@@ -54,6 +54,7 @@ export type {
 	ExtensionHandler,
 	ExtensionShortcut,
 	ExtensionUIContext,
+	ExtensionUIDialogOptions,
 	LoadExtensionsResult,
 	LoadedExtension,
 	MessageRenderer,