Motivation for the change, related issues

Currently, WordPress Playground can only fetch git:directory resources from public Github repositories, private ones are not supported.

Related: #2182.

This PR adds Github OAuth authentication support for private repositories:

• Load blueprints that reference private GitHub repositories
• Fetch a token via Github OAuth and a user displayed dialog
• Store the credentials (in memory or localStorage in dev mode, as previously)

Note: Because it uses Github OAuth and thus requires a Github user, this is not intended for demoing plugins that reside in private repos, rather for developers to run code in Playground that resides in their own private repositories.

Implementation details

Authentication Flow

• Added token storage and authentication helpers to git-sparse-checkout.ts
• Git protocol functions now send HTTP Basic Auth headers for Github URLs
• Throws GitHubAuthenticationError on 401/403 responses

UI

• New modal prompts users to authenticate when accessing private repos

Technical Details

• Changed OAuth scope from public_repo to repo
• Full page reload after OAuth to retry blueprint with new token

Testing Instructions (or ideally a Blueprint)

Test with a private repository:

  {
    "steps": [{
      "step": "installPlugin",
      "pluginData": {
        "resource": "git:directory",
        "url": "https://github.com/YOUR-USERNAME/YOUR-PRIVATE-REPO",
        "ref": "HEAD"
      }
    }]
  }

Expected: Modal prompts for GitHub auth → redirects to GitHub → returns and loads repo successfully without showing modal again.

Verify: Public repos still work without auth, token persists in dev mode (localStorage), no modal persistence issues.