sync-branches: configure git credentials before merge/push step

Add git identity (user.name, user.email) and authenticated remote URL
configuration before the Node.js merge script runs. Without these,
merge commits fail when git identity is not set, and push/fetch
operations fail when the remote URL lacks authentication.

Follows the same pattern used in workflows/autoloop.md for token-based
authentication via GITHUB_TOKEN env var.

Agent-Logs-Url: https://github.com/githubnext/autoloop/sessions/2ca3528c-1a17-4fba-9bc7-76da39c33eb9

Co-authored-by: mrjf <180956+mrjf@users.noreply.github.com>

Author: Copilot

workflows/sync-branches.md

@@ -23,8 +23,12 @@ steps:
   - name: Merge default branch into all autoloop program branches
     env:
       GITHUB_REPOSITORY: ${{ github.repository }}
+      GITHUB_TOKEN: ${{ github.token }}
       DEFAULT_BRANCH: ${{ github.event.repository.default_branch }}
     run: |
+      git config user.name "github-actions[bot]"
+      git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
+      git remote set-url origin "https://x-access-token:${GITHUB_TOKEN}@github.com/${GITHUB_REPOSITORY}.git"
       node - << 'JSEOF'
       const { execSync, spawnSync } = require('child_process');