fix(mirror): discover new versions from upstream sources

[CalvinAllen]

Problem

The mirror-binaries script currently reads from the local manifest file to determine what versions to sync to R2. This creates a chicken-and-egg problem where new upstream releases (like Node.js 20.20.0) are never discovered or mirrored.

Current flow:

1. mirror-sync reads local manifest → syncs those versions to R2
2. generate-manifests-from-r2 reads R2 → generates manifests

Since new versions aren't in the manifest, they're never synced, so they're never added to the manifest.

Solution

Refactor mirror-binaries to discover versions directly from upstream sources instead of reading the local manifest:

Node.js

• Source: https://nodejs.org/dist/index.json
• Single comprehensive source covering all platforms

Python

Multiple sources for comprehensive coverage:

• Official: https://www.python.org/ftp/python/
• Standalone builds: https://github.com/astral-sh/python-build-standalone/releases

Ruby

Multiple sources for comprehensive coverage:

• Windows: https://github.com/oneclick/rubyinstaller2/releases
• Other platforms: https://github.com/ruby/ruby-builder/releases

Implementation

1. Add upstream source fetching logic for each runtime
2. Build job list from upstream version data (not local manifest)
3. Compare against existing R2 contents
4. Download and upload any missing binaries
5. generate-manifests-from-r2 will then automatically pick up new versions

Acceptance Criteria

• mirror-binaries discovers new versions from upstream sources
• Node.js, Python, and Ruby all have their upstream sources implemented
• New releases are automatically mirrored within 24 hours of the daily cron
• Existing R2 content is preserved (no re-downloading)