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

[YoniMelki]

Summary

• Switch release.yml from commit-subject markers to comparing .codex-plugin/plugin.json (cross-checked with package.json) against the latest strict vX.Y.Z tag.
• Dedicated PR-time validate-version.yml (job id validate-version, contents: read); removed the duplicate inline check from pr.yml.
• Bump 0.1.2 → 0.1.3 in both manifests so merge is a real release.

Part of AX-2134.

Landing

Merging cuts v0.1.3. After that, every merge to main must bump both manifests or Release fails by design.

Test plan

• Confirm validate-version.yml catches plugin.json vs package.json drift and non-X.Y.Z versions.
• Confirm merge creates GitHub Release v0.1.3 with release.zip.
• Confirm a follow-up merge without a bump fails with “already released”.