Fix minimum token permissions for workflows that perform git push

[Copilot]

This PR adds the required contents: write permission to GitHub workflows that perform git push operations.

Changes Made

• auto-merge-repo-staging.yml: Added contents: write permission to the action job
• auto-merge-staging-main.yml: Added contents: write permission to the action job

Both workflows execute npm scripts (do-sandbox-repo-merge and do-sandbox-staging-merge) that ultimately call the pushToBranch function in the sandbox-tools/merge-repos scripts, which performs git push operations to update the repository.

Why This Fix is Needed

Without explicit permissions, GitHub Actions workflows default to read-only access. These workflows need write access to push changes back to the repository, but were missing the required contents: write permission.

Implementation Details

Each permission line includes the trailing comment "required for pushing changes" as specified in the issue requirements. The changes are minimal and follow GitHub Actions best practices for token permissions.

permissions:
  contents: write  # required for pushing changes

Fixes #21.

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