ci: tag main from extension.yml version when missing

[dango85]

Summary

Adds Tag version on main (.github/workflows/tag-version.yml): when a push to main changes extension.yml, the workflow reads extension.version, and if git tag v{version} does not exist yet, it creates an annotated tag and pushes it.

Why

Merging a release/docs PR does not create tags by default. catalog-entry.json and install docs use .../archive/refs/tags/vX.Y.Z.zip, so the tag must exist on GitHub. This automates that for future bumps where extension.yml is part of the merge.

Behavior

• Trigger: push to main, path extension.yml only.
• Idempotent: If v{version} already exists (e.g. manual gh release create), the job exits without failing.
• Does not create a GitHub Release; only the git tag (archive URL still works).

Follow-up (optional)

A separate workflow on push tags v* could call gh release create for release notes—only if you want Releases every time, not just tags.