feat: add plan file support and custom packages
- Add --out flag to plan command to save plan as JSON (defaults to coolpack.json)
- Add --packages flag to plan/build/prepare for custom APT packages
- Add --plan flag to build/prepare to use existing plan file
- Auto-detect coolpack.json in project root for build/prepare
- Add COOLPACK_PACKAGES env var support (comma-separated)
- Separate custom_packages from apt_packages in metadata
- Update generator to merge native and custom packages
- Update documentation with new features
Full Changelog: v0.0.1...v0.0.2