chore: enable automatic CHANGELOG generation via Changesets

Switch from manual CHANGELOG management to @changesets/changelog-github.
- Update .changeset/config.json to use @changesets/changelog-github
- Pass GITHUB_TOKEN to changeset version step in release workflow
  (@changesets/changelog-github needs it to fetch PR metadata for links)
- Fix git add to include CHANGELOG.md (pre-existing bug — changeset version
  modifies CHANGELOG.md but it was never staged in the commit step)
- Fix stale v3.0.6 '(Unreleased)' label — published to npm August 25, 2025

Author: kdaviduik

.changeset/config.json

@@ -1,6 +1,6 @@
 {
   "$schema": "https://unpkg.com/@changesets/config@3.1.1/schema.json",
-  "changelog": false,
+  "changelog": ["@changesets/changelog-github", { "repo": "Shopify/buy-button-js" }],
   "commit": false,
   "fixed": [],
   "linked": [],

.github/workflows/npm-release.yml

@@ -51,6 +51,8 @@ jobs:
       - name: Update package version and remove changesets
         if: env.changesets_found == 'true'
         run: pnpm exec changeset version
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Publish to NPM
         if: env.changesets_found == 'true'
@@ -69,6 +71,6 @@ jobs:
         run: |
           git config --global user.name 'github-actions[bot]'
           git config --global user.email 'github-actions[bot]@users.noreply.github.com'
-          git add package.json .changeset
+          git add package.json .changeset CHANGELOG.md
           git commit -m "Update package version & delete changesets"
           git push