Bump actions/checkout to v6 and replace EndBug/add-and-commit with git commands

Fixes Node.js 20 deprecation warnings. Both actions/checkout@v4 and
EndBug/add-and-commit@v9 use Node 20. Checkout bumped to v6 (Node 22),
and add-and-commit replaced with plain git commands.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hua7450

.github/workflows/push.yml

@@ -13,7 +13,7 @@ jobs:
     if: "!contains(github.event.head_commit.message, 'Update PolicyEngine Claude')"
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Bump version
         run: python .github/bump_version.py
@@ -22,11 +22,10 @@ jobs:
         run: python .github/build_changelog.py
 
       - name: Commit and push
-        uses: EndBug/add-and-commit@v9
-        with:
-          message: "Update PolicyEngine Claude"
-          default_author: github_actions
-          add: |
-            .claude-plugin/marketplace.json
-            CHANGELOG.md
-            changelog.d/
+        run: |
+          git config user.name "github-actions[bot]"
+          git config user.email "github-actions[bot]@users.noreply.github.com"
+          git add .claude-plugin/marketplace.json CHANGELOG.md changelog.d/
+          git diff --staged --quiet && echo "No changes to commit" && exit 0
+          git commit -m "Update PolicyEngine Claude"
+          git push

.github/workflows/test.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Require changelog fragment
         run: |
@@ -34,7 +34,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Check JSON syntax
         run: |