fix: add missing @changesets/changelog-github devDependency and CHANGELOG date

Two items from the PR 4a plan that were missed in the original commit:

1. @changesets/changelog-github was referenced in .changeset/config.json
   but never declared in package.json devDependencies. The lockfile had
   the specifier (from pnpm install), so CI's --frozen-lockfile saw a
   mismatch and failed.

2. CHANGELOG.md v3.0.6 date was still "(Unreleased)" — v3.0.6 was
   published to npm on August 25, 2025.

Author: kdaviduik

CHANGELOG.md

@@ -1,5 +1,5 @@
 # Changelog
-### v3.0.6 (Unreleased)
+### v3.0.6 (August 25, 2025)
 - Add title attribute to iframes for improved accessibility ([#919](https://github.com/Shopify/buy-button-js/pull/919))
 
 ### v3.0.5 (July 2, 2025)

package.json

@@ -72,6 +72,7 @@
     "@babel/plugin-transform-template-literals": "7.4.4",
     "@babel/plugin-transform-typeof-symbol": "7.2.0",
     "@babel/preset-typescript": "^7.27.1",
+    "@changesets/changelog-github": "^0.6.0",
     "@changesets/cli": "^2.28.1",
     "@shopify/js-uploader": "https://github.com/Shopify/js-uploader.git",
     "@types/jest": "^30.0.0",