feat: unified hook notifier, agent auto-detection, terminal transition UI

- Add hooks/notify.ps1 as single hook handler for all events
- Refactor settings.local.json to use notify.ps1 instead of inline PS
- Add Notification hook, auto-detect agent from session_id/transcript
- Rename agent 'main' to 'claude' across server routes and terminal
- Add loading overlay and error state for terminal switching transitions
- Add transitionError ref to useTranscriptDebug composable

server/routes/transcript.ts

@@ -22,7 +22,7 @@ export function handleTranscriptSessions(): Response {
 export function handleTranscriptActive(req: Request, url: URL): Response {
   if (req.method !== 'GET') return errorResponse('Method not allowed', 405)
 
-  const agent = url.searchParams.get('agent') || 'main'
+  const agent = url.searchParams.get('agent') || 'claude'
   const activeSession = getActiveSession(agent)
 
   const sessionId = activeSession?.sessionId