[Feat] Add per-task context folder file watching via fs.watch

[samzong]

Summary

Add live file watching for per-task context folders. When users add context folders to a task, file changes (create/modify/delete) are detected via fs.watch and the file list auto-refreshes — no manual re-add needed.

Type of change

• [Feat] new feature

Why is this needed?

Context folder file lists were static — only refreshed on manual actions like re-adding the folder. Users had no way to see newly created files without closing and re-opening the folder. This makes the context folder feature feel live: create or delete a file and the count updates within ~1 second.

What changed?

• New file-watcher.ts module in main process: fs.watch({ recursive: true }) manager with 500ms debounce, max 10 watchers per task, silent error handling (EPERM/ENOENT), initial synthetic event suppression, and full cleanup on app quit via unwatchAll()
• Two new IPC handlers in context-handlers.ts: context:watch-folder and context:unwatch-folder
• Three new preload bridge methods: watchContextFolder, unwatchContextFolder, onContextFilesChanged
• ChatInput watcher lifecycle: watch on folder add, unwatch on remove, switch watchers on task change, cleanup on unmount
• Race-condition guard (refreshIdRef) prevents stale async IPC results from overwriting fresh ones
• Live text file count displayed next to context folder pills in ChatInput

Architecture impact

• Owning layer: main (file-watcher.ts, context-handlers.ts) / preload (bridge) / renderer (ChatInput.tsx)
• Cross-layer impact: yes — new IPC channel context:files-changed flows main → renderer via sendToWindow (reuses existing window-utils.ts utility)
• Invariants touched from docs/architecture-invariants.md: none — context folders are client-only state, not persisted to DB, no message persistence paths affected
• Why those invariants remain protected: file watching only triggers UI refresh (file count); no writes to DB, no message creation, no gateway interaction

Linked issues

N/A

Validation

• pnpm lint
• pnpm test
• pnpm build
• Manual smoke test
• Not run

Commands, screenshots, or notes:

pnpm check (lint + architecture + format + typecheck + test) — all passed, 88 tests green

Screenshots or recordings

N/A — file count label is a plain text span next to existing folder pills, no layout or design-system changes.

Release note

• No user-facing change. Release note is NONE.
• User-facing change. Release note is included below.

Context folders now auto-refresh when files are created, modified, or deleted. A live file count is shown next to each folder pill.

Checklist

• The PR title uses at least one approved prefix: [Feat], [Fix], [UI], [Docs], [Refactor], [Build], or [Chore]
• The summary explains both what changed and why
• Validation reflects the commands actually run for this PR
• Architecture impact is described and references any touched invariants
• Cross-layer changes are explicitly justified
• The release note block is accurate

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