Add Copilot setup workflow to stabilize failing copilot Actions job

[Copilot]

The copilot GitHub Actions run was failing in the cloud-agent execution path, with no repository-specific setup in place. This PR adds a dedicated Copilot setup workflow so agent sessions start from a predictable Go environment instead of ad hoc runtime setup.

• Copilot agent bootstrap

  • Added .github/workflows/copilot-setup-steps.yml with the required copilot-setup-steps job.
  • Configured triggers for workflow_dispatch, and changes to the setup workflow on push/pull_request.

• Deterministic Go environment for Copilot sessions

  • Checks out repository content.
  • Installs Go using the version declared in go.mod.
  • Pre-fetches module graph via go mod download.

jobs:
  copilot-setup-steps:
    runs-on: ubuntu-latest
    permissions:
      contents: read
    steps:
      - uses: actions/checkout@v5
      - uses: actions/setup-go@v5
        with:
          go-version-file: go.mod
      - run: go mod download