feat: auto-fetch head-ref from GitHub Actions in detached HEAD state

[runningcode]

Summary

• Add get_github_head_ref() function to automatically detect head branch from GitHub Actions environment variables
• Integrate with upload command fallback chain to resolve detached HEAD scenarios
• Use GITHUB_HEAD_REF for automatic head-ref detection in PR workflows

See context reference: https://docs.github.com/en/actions/reference/workflows-and-actions/contexts

Context

Resolves EME-367 - Auto-fetch correct head-ref value in detached HEAD state using GitHub Actions env variables. Follows the same pattern as recent work on base SHA auto-fetching (#2799).

🤖 Generated with Claude Code

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Note

Adds get_github_head_ref() (reads GITHUB_HEAD_REF in pull_request) and makes build upload prefer it before git head ref to handle detached HEAD.

• Build Upload:
  • Update src/commands/build/upload.rs head-ref resolution to first use get_github_head_ref(); fall back to git_repo_head_ref.
• VCS Utils:
  • Add get_github_head_ref() to read GITHUB_HEAD_REF when GITHUB_EVENT_NAME is pull_request with debug logging.
  • Export and import updated in utils::vcs and upload command.

^{Written by Cursor Bugbot for commit 332fe76. This will update automatically on new commits. Configure here.}

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EME-367 Auto-fetch correct head-ref value in detached HEAD state using GitHub Actions env variables