Implement `cora watch` — background file watcher (Phase 2)

[ajianaz]

Goal

Standalone cora watch command using notify crate for real-time file system watching.

Spec

cora watch                      # Watch current project, auto-reindex on change
cora watch --debounce 500ms     # Custom debounce window
cora watch --git-only           # Only trigger on git-tracked files
cora watch --verbose            # Log every event + reindex

Implementation

1. Add notify crate dependency (cross-platform: inotify/FSEvents/kqueue)
2. Watch project root recursively
3. Debounce: collect events in 500ms window, then batch reindex
4. Git-aware: ignore untracked files (.gitignore)
5. Reindex: use existing incremental pipeline
6. Signal handling: SIGTERM/SIGINT → clean shutdown

Dependencies

• notify crate (~50KB compiled)
• No new heavy deps

Acceptance

• cora watch starts and detects file changes
• Modifying a .rs file triggers reindex within 1s
• .gitignore-ed files are ignored
• Clean shutdown on Ctrl+C

Estimated Effort

3-5 days

[ajianaz]

Phase 1 (auto-reindex on serve): #434