Fix GitHub CLI token environment variable in auto-merge workflow

[Copilot]

The auto-merge GitHub Actions workflow was failing with the error:

gh: To use GitHub CLI in a GitHub Actions workflow, set the GH_TOKEN environment variable. Example:
  env:
    GH_TOKEN: ${{ github.token }}
Error: Process completed with exit code 4.

The issue was that the workflow was using GITHUB_TOKEN as the environment variable name, but the GitHub CLI (gh command) specifically requires GH_TOKEN when running in GitHub Actions.

This PR fixes the environment variable names in two places in .github/workflows/auto-merge.yml:

• The gh pr review --approve command
• The gh pr merge --auto --squash --delete-branch command

Both now correctly use GH_TOKEN instead of GITHUB_TOKEN, following the same pattern already used successfully in the post-release-version-bump.yml workflow.

The fix is minimal and surgical - only 2 lines changed in 1 file, preserving all existing functionality while resolving the build failure.

Fixes #37.

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