Add pkg/ — distribution wrappers for npm, PyPI, Homebrew, Scoop, Winget, Chocolatey, AUR

Introduces pkg/ as a self-contained home for all package registry wrappers.
Each subdirectory is a ready-to-publish package that downloads the appropriate
pre-built binary from GitHub Releases — no source compilation at install time.

- pkg/npm/          — postinstall downloads binary; bin.js shim for CLI
- pkg/pypi/         — lazy download on first run via _cli.py; pipx-friendly
- pkg/homebrew/     — multi-arch formula (darwin/linux × arm64/amd64)
- pkg/scoop/        — Windows bucket manifest with autoupdate + checksum
- pkg/winget/       — v1.6.0 manifests (version + installer + locale)
- pkg/chocolatey/   — nuspec + install/uninstall scripts
- pkg/aur/          — PKGBUILD + .SRCINFO for x86_64 and aarch64

All hashes are real SHA-256 checksums from the v0.6.0 release.