chore: sync the plugin version from package.json on release

`npm run version` now runs `changeset version` and then
`scripts/sync-plugin-version.mjs`, which copies the new version into
`.claude-plugin/plugin.json`. The release workflow calls `npm run version`
instead of `npx changeset version`, so the version PR carries both files.

Also closes the drift this replaces: `plugin.json` was manually bumped to
1.2.1 while `package.json` stayed at 1.2.0. `package.json` moves up to
1.2.1 so the plugin version never goes backwards.

`npm run check-plugin-version` reports drift without writing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "mattpocock-skills",
-  "version": "1.2.0",
+  "version": "1.2.1",
   "private": true,
   "description": "Matt Pocock's agent skills for real engineering",
   "repository": {
@@ -10,7 +10,8 @@
   "license": "MIT",
   "scripts": {
     "changeset": "changeset",
-    "version": "changeset version"
+    "version": "changeset version && node scripts/sync-plugin-version.mjs",
+    "check-plugin-version": "node scripts/sync-plugin-version.mjs --check"
   },
   "devDependencies": {
     "@changesets/changelog-github": "^0.7.0",