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

[YoniMelki]

Summary

• Switch release.yml from commit-subject markers to comparing .devin-plugin/plugin.json against the latest vX.Y.Z tag.
• Fail loudly when the version is already released or older than the latest tag.
• Keep layout validation, zip packaging, and atomic tag+release creation.

Part of AX-2134.

Test plan

• Confirm release.yml triggers on push to main and does not parse commit messages.
• Confirm a merge without a version bump fails with an "already released" error.
• Confirm a newer version would still produce release.zip and a GitHub Release.

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