Deprecate github-proxy.com with automatic migration to native resources

[adamziel]

Summary

github-proxy.com is being shut down. This PR provides a graceful migration path for existing Blueprints that reference github-proxy.com URLs.

When a Blueprint contains a URL resource pointing to github-proxy.com, it is now automatically rewritten to native Blueprint resources with a console warning prompting users to update their Blueprints.

Migration mapping

github-proxy.com pattern	Migrates to
?repo=owner/name	zip wrapping git:directory at HEAD
?repo=...&branch=trunk	zip wrapping git:directory with branch ref
?repo=...&pr=123	zip wrapping git:directory with refs/pull/123/head
?repo=...&commit=abc	zip wrapping git:directory with commit ref
?repo=...&release=v1.0	zip wrapping git:directory with tag ref
?repo=...&directory=subdir	zip wrapping git:directory with path
?repo=...&release=v1.0&asset=file.zip	url to GitHub releases download
?repo=...&release=latest&asset=file.zip	url to GitHub /releases/latest/download/
https://github-proxy.com/https://...	url with the inner GitHub URL

New zip resource type

A new resource type that wraps file or directory resources and outputs them as ZIP files. This maintains semantic compatibility with github-proxy.com which always returned ZIP files.

{
  "resource": "zip",
  "inner": {
    "resource": "git:directory",
    "url": "https://github.com/owner/repo",
    "ref": "main"
  }
}

Other changes

• Fixed ZIP encoding to use 0-based disk numbering (was causing PHP's ZipArchive to reject files with "multi-disk archive" error)
• Updated documentation to use git:directory instead of github-proxy.com examples
• The /proxy redirect now points to the resources documentation
• Local dev server now proxies /cors-proxy/ requests through Vite to avoid Chrome's Private Network Access restrictions

Test plan

• Run a Blueprint with a github-proxy.com URL and verify it works with a deprecation warning
• Test all github-proxy.com URL patterns (branch, PR, commit, release, directory, asset)
• Verify the zip resource type works with installPlugin and installTheme steps
• Check that existing git:directory resources still work as expected

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