Implement cora watch — background file watcher (Phase 2) #436

Goal

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

Status

• ✅ cora index --watch already exists — inotify-based file watching with auto-reindex
• ⏳ Standalone cora watch command — not yet promoted to top-level

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 --filter 'src/**/*.rs'  # Watch specific patterns

Remaining Tasks

• Promote to standalone cora watch CLI command
• Add --debounce, --git-only, --filter flags
• MCP push notification on reindex (complement Epic: Background Watcher #433 Phase 2)