AX-1735: Add tag/release mechanism

[YoniMelki]

Summary

• Adds VERSION file at repo root (currently 0.5.4) as the single source of truth for versioning
• Adds release.yml workflow: triggers on push to main when commit message contains [major], [minor], or [patch]; bumps the version, updates plugin.json and marketplace.json, commits, tags, and creates a GitHub Release with a zip artifact
• Adds validate-version.yml workflow: runs on PRs to main and fails if VERSION, plugin.json.version, and marketplace.json.metadata.version are out of sync

Part of Epic AX-1733.