Clean up stale release branch before Create Release PR retry

[amirejaz]

Summary

When the Create Release PR workflow fails after creating the release/vX.Y.Z branch but before opening the PR (e.g. helm-docs or file update failure), the branch remains. A retry fails with "Reference already exists" because releaseo cannot create the same branch again.

This change adds a pre-step that:

1. Computes the target release branch from the VERSION file and bump_type input
2. Checks whether there is an open PR for that branch
3. Deletes the branch only when there is no open PR (treats it as stale from a failed run)
4. Fails with an explanatory message when a release PR already exists, so the branch is not deleted

Type of change

• Bug fix
• New feature
• Refactoring (no behavior change)
• Dependency update
• Documentation
• Other (describe):

Test plan

• Unit tests (task test)
• E2E tests (task test-e2e)
• Linting (task lint-fix)
• Manual testing (describe below)

Manually verified the cleanup logic: branch name derivation matches releaseo (release/v{major}.{minor}.{patch}), and gh pr list/gh api usage is valid.

Changes

File	Change
.github/workflows/create-release-pr.yml	Add "Clean up stale release branch" step before releaseo
.claude/skills/toolhive-release/references/WORKFLOW-REFERENCE.md	Document the fix in troubleshooting

Does this introduce a user-facing change?

No. Improves workflow robustness for release automation only.

Special notes for reviewers

The cleanup step uses secrets.GITHUB_TOKEN (via GH_TOKEN) for gh CLI, separate from secrets.RELEASE_TOKEN used by releaseo. contents: write is sufficient for deleting refs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.56%. Comparing base (3aefbc8) to head (3aadc6d).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4035      +/-   ##
==========================================
- Coverage   68.62%   68.56%   -0.06%     
==========================================
  Files         444      444              
  Lines       45222    45222              
==========================================
- Hits        31034    31008      -26     
- Misses      11791    11817      +26     
  Partials     2397     2397              

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