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

hooks/notify.ps1

@@ -0,0 +1,7 @@
+# Unified hook notifier - forwards all hook events to agent-ui server
+# Claude Code pipes JSON via stdin with session_id, transcript_path, etc.
+# The server routes to the correct agent automatically.
+$b = [Console]::In.ReadToEnd()
+try {
+  Invoke-RestMethod -Uri 'http://localhost:4101/api/claude-hook' -Method POST -Body $b -ContentType 'application/json' -TimeoutSec 3 | Out-Null
+} catch {}