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

[YoniMelki]

Summary

• Switch release.yml from tag-push to push-to-main, comparing VERSION (cross-checked with gradle.properties) against the latest strict vX.Y.Z tag.
• Add concurrency (cancel-in-progress: false). Tag is created atomically via gh release create --target.
• No version bump. Repo has zero tags; VERSION is 0.2.0. Merging is the intentional first GitHub Release of v0.2.0.
• Zip filename is asserted to contain the version; partial releases are deleted only if the gate had already passed.

Part of AX-2134.

Landing

Merging cuts v0.2.0 (Gradle buildPlugin zip). Marketplace publish is unchanged (workflow_dispatch). After this, every merge must bump VERSION + gradle.properties or Release fails.

Test plan

• Confirm release.yml no longer triggers on tag push.
• Confirm merge builds the plugin and creates GitHub Release v0.2.0.
• Confirm concurrency is cancel-in-progress: false.