AX-1736: Add tag/release mechanism

[YoniMelki]

Summary

• Adds VERSION file (currently 1.0.4) as the single source of truth for the plugin version
• Adds release.yml workflow: triggered on pushes to main that include [major], [minor], or [patch] in the commit message — bumps VERSION, plugin/.claude-plugin/plugin.json, and marketplace.json, commits + tags, then creates a GitHub Release with a zip artifact
• Adds validate-version.yml workflow: runs on PRs to main, fails if VERSION is out of sync with either JSON file

Test plan

• Merge a commit to main with [patch] in the message and verify the release workflow runs, bumps to 1.0.5, creates a tag v1.0.5, and publishes a GitHub Release
• Open a PR where VERSION doesn't match one of the JSON files and confirm validate-version fails with a clear error
• Open a PR with all versions in sync and confirm validate-version passes

Part of Epic AX-1733.