fix: add OS version to cache key to prevent binary incompatibility (#716)

## 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

action.yml

@@ -89,6 +89,8 @@ outputs:
     description: "The path to the installed uvx binary."
   cache-hit:
     description: "A boolean value to indicate a cache entry was found"
+  cache-key:
+    description: "The cache key used for storing/restoring the cache"
   venv:
     description: "Path to the activated venv if activate-environment is true"
 runs: