Debian unstable and testing don't have VERSION_ID in /etc/os-release.
This change falls back to VERSION_CODENAME when VERSION_ID is missing,
producing cache keys like 'debian-sid' for unstable.
Fixes#773
## Summary
- Adds OS name and version (e.g., `ubuntu-22.04`, `macos-14`,
`windows-2022`) to cache keys to prevent binary incompatibility when
GitHub updates runner images
- Fixes issue where cached uv binaries compiled against older
glibc/library versions fail on newer runner OS versions
## Changes
- Added `getOSNameVersion()` function to `src/utils/platforms.ts` with
OS-specific detection for Linux (via `/etc/os-release`), macOS (Darwin
kernel version mapping), and Windows
- Updated cache key format to include OS version, bumped `CACHE_VERSION`
to `"2"`
- Added `cache-key` output to expose the generated cache key for
debugging
- Added `test-cache-key-os-version` job testing across multiple OS
versions
- Updated `docs/caching.md` with cache key documentation
Closes#703
