AX-2134: unify release trigger on version-file diff

[YoniMelki]

Summary

• Switch release.yml from commit-subject [major|minor|patch] markers to comparing .claude-plugin/plugin.json against the latest strict vX.Y.Z tag.
• Fail loudly when the version is already released or older than the latest tag (no silent green no-op).
• Bump plugin version 0.2.21 → 0.2.22 so merging this PR is a real release, not a red “already released” run.
• PR-time X.Y.Z shape check in validate.yml; cleanup of a partial GitHub Release only if the gate had already passed.

Part of AX-2134.

Landing

Merging cuts v0.2.22. After that, every merge to main must bump the manifest or Release fails by design.

Test plan

• Confirm release.yml does not parse commit messages.
• Confirm merge creates GitHub Release v0.2.22 with release.zip.
• Confirm a follow-up merge without a bump fails with “already released”.