feat(ci): auto-merge changesets release PRs once checks pass

The version-packages PR opened by changesets/action was previously
opened and waited for a human merge. Enable auto-merge on it via
gh pr merge --auto so the release pipeline becomes fire-and-forget:
once CI is green, the PR squash-merges and the next release-npm.yml
run publishes packages.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: counterposition

.github/workflows/release-npm.yml

@@ -37,10 +37,18 @@ jobs:
         run: pnpm run check
 
       - name: Create release PR or publish packages
+        id: changesets
         uses: changesets/action@6a0a831ff30acef54f2c6aa1cbbc1096b066edaf # v1.7.0
         with:
           version: pnpm run version:packages
           publish: pnpm run release:npm
           createGithubReleases: true
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Enable auto-merge on release PR
+        if: steps.changesets.outputs.pullRequestNumber != ''
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          PR_NUMBER: ${{ steps.changesets.outputs.pullRequestNumber }}
+        run: gh pr merge --auto --squash "$PR_NUMBER"