feat: Auto-sync file watcher daemon

[ajianaz]

Description

Background file watcher that auto-updates the symbol index on file changes.

• Watches project directory using OS-native file events (inotify/kqueue/FSEvents)
• Debounced sync (2-second quiet window, configurable)
• Filters to source files only (respect .gitignore)
• Index stays fresh as you code — no manual cora index needed

Why

CodeGraph has auto-sync as a core feature. Without it, the index goes stale between manual runs. This makes code intelligence feel seamless.

Scope

• Cross-platform file watcher (notify crate or similar)
• Debounce logic with configurable window
• Incremental re-index of changed files only
• Status command: cora index --status (pending sync count)

Depends On

• feat: cora index — Symbol index & SQLite persistence #264 cora index

Strategy Ref

Code intelligence strategy #1 from competitive analysis (Phase 3).

Note

Lower priority — can ship v0.6 without this (manual cora index is fine initially).