ci: release workflow auto-creates publish branch

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What

The publish step assumed the publish branch already existed, running git fetch origin <publish-branch> and git worktree add … origin/<publish-branch> unconditionally. On a repo's first release that branch doesn't exist yet, so the run died with:

fatal: couldn't find remote ref publish

By then it had already pushed the release commit to the dev branch and created the version tags, leaving a half-finished release.

The step now checks whether the branch exists on the remote first. If it does, behavior is unchanged. If it doesn't, the worktree is seeded from the released commit and the step's existing git push origin HEAD:<publish-branch> creates the branch.

Seeding from HEAD rather than an empty orphan branch is deliberate: merge-feed --into _publish/DependencyControl.json exits non-zero when that file is missing, and the step's rsync excludes DependencyControl.json, so an orphan branch would fail one step later instead.

Why it matters beyond this repo

reusable-publish-release.yml is the building block third-party package repos call, so every one of them would hit this on their first release. The README's setup step told them to create the branch by hand, which is no longer necessary.

Testing

Not exercised by a real run. This surfaced while releasing DependencyControl 0.7.0, and that release was unblocked by creating the publish branch manually, so the new bootstrap path hasn't run in CI yet. It will first execute on the next package repo that releases without a publish branch.

🤖 Generated with Claude Code

[github-actions]

Test results — macos-latest ✅1054 · ubuntu-latest ✅1054 · windows-latest ✅1051

Summary

Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
3162	3159	0	3	0	0	1m 39s

_{🎉 No failed tests in this run.}

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