Fix live file tree startup latency

[backnotprop]

Summary

• delay the file-tree SSE subscription until the first successful tree snapshot has loaded
• wait for the initial regular-folder batch to settle before opening the live stream, so one fast folder cannot start watcher setup while another initial folder is still loading
• keep the existing chokidar-based server watchers unchanged
• run the file-browser DOM hook tests in CI so the startup-order behavior is covered

Verification

• DOM_TESTS=1 bun test packages/ui/hooks/useFileBrowser.test.tsx
• bun test packages/server/reference-watch.test.ts apps/pi-extension/server/file-browser-watch.test.ts
• bun run typecheck
• bun test
• bun run build:hook
• bun run build:opencode