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

[YoniMelki]

Summary

• Switch release.yml from commit-subject markers to comparing plugins/jfrog/.cursor-plugin/plugin.json (cross-checked with marketplace.json) against the latest vX.Y.Z tag.
• Fail loudly on equal/older versions instead of a silent skip.
• Keep the existing PR-time validate-version.yml check and zip packaging.

Part of AX-2134.

Test plan

• Confirm release.yml triggers on push to main with no commit-message parsing.
• Confirm an unchanged version vs latest tag fails with "already released".
• Confirm plugin.json / marketplace.json cross-check still runs at release time.

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