AX-1734: Add tag/release mechanism

[YoniMelki]

Summary

• Adds VERSION file (currently 0.2.7) as the single source of truth for the plugin version
• Adds .github/workflows/release.yml: triggers on push to main when the commit message contains [major], [minor], or [patch]; bumps the version, updates VERSION and .claude-plugin/plugin.json, commits, tags, and creates a GitHub Release with a zipped artifact
• Adds .github/workflows/validate-version.yml: PR check that fails if VERSION and .claude-plugin/plugin.json.version are out of sync

How to trigger a release

Merge a PR whose commit message (or squash message) includes a bump tag, e.g.:

[minor] Add some new feature

The workflow picks up [major], [minor], or [patch] and increments accordingly.

Test plan

• Verify validate-version check passes on this PR (versions are consistent at 0.2.7)
• After merge, confirm release.yml is registered in Actions (no [major/minor/patch] tag in this commit, so no release fires)
• On a follow-up PR, use [patch] in the squash message and confirm the workflow creates v0.2.8

Closes AX-1734 / part of AX-1733