perf: incremental cross-window session watch (fix window-open stutter)

[yasithdev]

Problem

Opening a new remote (cshost) VS Code window briefly hangs the other already-open windows, even for a completely unrelated session.

It isn't SSH — remote windows are observe-only (reattachLiveSessions bails on a remote window). It's the cross-window state sync: watchSessions runs mergeFromDisk(sessions, readAllRecords()) on every record write (extensionStore.ts), re-reading and re-parsing every session file. Every open window runs this watcher, so one window writing a single record (e.g. registering its pid on open via mutateWindowPids) makes every other window synchronously re-read all session files and re-render. Cost scales O(windows × sessions) per write, on the extension-host event loop.

(Note: an async lock was considered and rejected — the per-file lock is rarely contended and isn't the bottleneck; the re-read fan-out is. Making the lock async would be a large async ripple that doesn't touch the actual cost.)

Fix

Use the filename fs.watch already provides and merge only the one record that changed (mergeRecord, undefined = deleted) instead of reading all of them. A short debounce coalesces the 2–3 raw events macOS fires per atomic temp+rename into a single read + render. readAllRecords/mergeFromDisk stay for the initial load and the rare platform that reports no filename.

Per-event work: O(sessions) → O(1). Unrelated windows stop re-reading and (via the existing per-window guard) stop re-rendering on writes that aren't theirs.

Test plan

• npm run check-types, npm run lint — clean
• npm test — 110/110 (added mergeRecord upsert/delete cases)
• End-to-end against the real filesystem + real write path (temp+rename): confirmed fs.watch reports the filename, one write reads only the one changed file (not all 5), and the debounce coalesces macOS's duplicate events to a single read + single render, with the record merged correctly.

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