Watch mode performance and usability improvements

Performance:
- Improve watch mode reindex speed by 50% (4s → 2s for single file changes)
- Remove JSON indentation in chunk files to reduce 5MB marshal overhead

Bug Fixes:
- Fix **/*.md glob pattern to match root-level files like README.md
- Previously only matched nested files (docs/guide.md)
- Users no longer need both *.md and **/*.md patterns

Other Changes:
- Add "Detected change" log for better watch mode feedback
- Clean up event filtering logic
- Maintain backward compatibility with existing configs

Full Changelog: https://github.com/mvp-joe/project-cortex/compare/v1.3.0...v1.3.1