Release v2.1.6

Rust rewrite: v2.1.6 release

- Complete Rust rewrite with modular workspace (5 crates)
- CLI: generate, validate, completion (bash/zsh/fish/powershell/elvish)
- Core: Config, validation, fix-it, Provider trait
- Providers: Local, AWS, Azure, GCP (feature-flagged)
- Templates: Handlebars-based rendering
- CI/CD: Updated for Rust (cargo test, clippy, fmt, cross-platform build)
- Release: v2.1.6, ZIP artifacts (linux-x86_64, windows-x86_64)
- Removed Python source, tests, build configs, packaging
- Updated docs: README.md, GUIDE.md, CHANGELOG.md