fix: retry the same unpublished tag; do not skip versions

[xeaser]

What

v0.5.1 was tagged but never got a GitHub Release; we jumped to v0.5.2. The v0.5.2 GoReleaser job published assets, then failed pushing Homebrew/Scoop/winget to main, so the packaging PR never opened. Auto-merge is also disabled on this repo, so gh pr merge --auto would fail that job even after a PR exists.

• Refuse to publish a newer stable tag while the previous stable tag has no GitHub Release.
• workflow_dispatch retries the same tag: checkout tag code, use origin/main:.goreleaser.yaml.
• Packaging bump warns if auto-merge is unavailable (PR stays open).
• Packaging bump skips when the tag is not the latest GitHub Release (backfilling 0.5.1 must not downgrade pins).

After merge: run Actions → release → v0.5.1 to publish the missing release.

How to test

• go test ./...
• Docs updated (CONTRIBUTING release section)

Checklist

• Commits are signed
• No secrets
• Does not add Copilot, Ink shell, Aspire, or npm install