Fix pip cache ordering: move checkout first, use setup-python built-in cache

[Copilot]

actions/cache ran before actions/checkout, so hashFiles(...) evaluated against an empty workspace — producing a constant cache key and making dependency caching effectively a no-op.

Changes

• Moved actions/checkout to be the first step so all subsequent steps have access to workspace files
• Replaced manual actions/cache with setup-python's built-in cache: pip — eliminates the ordering problem entirely and reduces boilerplate

- name: Checkout code
  uses: actions/checkout@v4

- name: Setup Python
  uses: actions/setup-python@v5
  with:
    python-version: '3.x'
    cache: 'pip'
    cache-dependency-path: |
      pyproject.toml
      requirements.txt
      setup.cfg
      setup.py

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.