feat(watch): standalone cora watch command with auto-reindex

[ajianaz]

What

Add standalone cora watch command that monitors file changes and auto-reindexes the project.

Why

Currently file watching is embedded inside cora index --watch with a simple 2-second poll loop. Issue #436 requests a proper standalone watcher with configurable debounce, git-only filtering, and glob pattern support — making it usable as a background daemon during active development.

How

• New module src/commands/watch.rs with run_watch() entry point
• Poll-based change detection (no new dependency like notify crate needed)
• Debounce window configurable via --debounce <ms> (default: 500ms)
• --git-only flag: only process files tracked by git ls-files
• --filter <glob>: limit to matching paths (e.g. src/**/*.rs)
• Skips hidden directories (.git, .cora), node_modules, target/
• Initial full index, then incremental reindex on detected changes
• Reuses existing index::index_project_with_skip() for consistency

CLI

cora watch [--debounce 500] [--git-only] [--filter 'src/**/*.rs']

Testing

• cargo test --features tree-sitter — 898 tests, 0 failures
• cargo clippy --all-targets --features tree-sitter -- -D warnings — 0 warnings
• cargo fmt -- --check — clean
• 5 new unit tests: walk_files, detect_changes (empty, source, non-source filter), git-tracked fallback

Related Issues

Closes #436

Checklist

• Code follows project conventions (CONTRIBUTING.md)
• Tests added for new functionality
• No clippy warnings
• Commit message follows Conventional Commits