Stabilize Copilot Actions by normalizing git object state before request processing

[Copilot]

The Copilot workflow was failing in the Dependabot Checker phase due to git object resolution issues in the runtime clone (unable to normalize alternate object path) and inconsistent local availability of main.
This PR adds a Copilot preflight setup workflow to normalize repository object storage and ensure the base branch ref is present before agent tools run.

• Root-cause guardrail: git alternates normalization

  • Added .github/workflows/copilot-setup-steps.yml with a copilot-setup-steps job.
  • Detects .git/objects/info/alternates, repacks objects into the local repo, removes alternates, and verifies connectivity.

• Base-branch availability for tooling

  • Explicitly fetches origin/main into refs/remotes/origin/main during setup to avoid base-commit lookup drift in downstream checks.

• Copilot-specific bootstrap wiring

  • Uses actions/checkout@v5.
  • Limits permissions to contents: read.
  • Triggers on workflow dispatch and on changes to the setup workflow file (push/PR) for easy iteration.

- name: Normalize git object store for Copilot tools
  run: |
    if [[ -f .git/objects/info/alternates ]]; then
      git repack -a -d
      rm -f .git/objects/info/alternates
      git fsck --connectivity-only
    fi

- name: Ensure main is available locally
  run: git fetch --no-tags --prune origin +refs/heads/main:refs/remotes/origin/main