[kpal81xd]

Follow-up to #108 and #109, which each added a plugin version-pin check: scripts/check-plugin-pin.mjs in CI, and scripts/sync-plugin-version.mjs --check, which nothing called. This keeps one.

• sync-plugin-version.mjs stays, because the Publish workflow already uses it to write the pins. Its --check mode now also covers prereleases: every plugin pin must be the same stable version, instead of the check being skipped.
• CI's Plugin job runs node scripts/sync-plugin-version.mjs --check.
• scripts/check-plugin-pin.mjs is removed.

I ran --check on a copy of package.json and plugin/ for each case:

Case	Result
stable, pins match	pass
stable, package bumped to 0.7.2	fail, lists the three stale files
0.8.0-beta.0, pins on 0.7.1	pass ("plugin stays pinned to 0.7.1")
0.8.0-beta.0, README drifted to 0.7.0	fail
0.8.0-beta.0, beta leaked into .mcp.json	fail
0.8.0-beta.0, README pin removed	fail ("no version")

Smoke test

1. Open this PR's Checks tab. The Plugin job's "Check plugin version pin" step prints plugin version matches 0.7.1.