fix: create GitHub releases on version bump

Add `publish: npx changeset tag` to the release workflow and enable
tagging for private packages in the changesets config, so merging a
Version PR creates a git tag and GitHub release.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.changeset/config.json

@@ -5,6 +5,7 @@
     { "repo": "mattpocock/skills" }
   ],
   "commit": false,
+  "privatePackages": { "version": true, "tag": true },
   "fixed": [],
   "linked": [],
   "access": "restricted",

.github/workflows/release.yml

@@ -30,6 +30,7 @@ jobs:
         uses: changesets/action@v1
         with:
           version: npx changeset version
+          publish: npx changeset tag
           commit: "chore: version skills"
           title: "chore: version skills"
         env: