[orbit-agent] Swap pip → uv in CI workflow(s)

[haasonsaas]

Summary

• Replace python -m pip install -U pip + pip install -r requirements-dev.lock with uv pip install --system -r requirements-dev.lock in .github/workflows/ci.yml.
• Add astral-sh/setup-uv@08807647e7069bb48b6ef5acd8ec9567f424441b (v8.1.0, SHA-pinned) after Set up Python; applies to both matrix legs (3.11, 3.12).
• scripts/bootstrap_venv.sh is intentionally left alone — Wave 3, out of scope.

Cross-ref: evalops/maestro-internal#1492

Test plan

• CI matrix run on this PR is green for python-version 3.11 and 3.12.
• black --check, ruff check, and pytest -q all pass against uv-installed deps.

🤖 Generated with Claude Code

[cursor]

PR Summary

Low Risk
Low risk: only changes the CI dependency installation mechanism; failures would surface as CI breakage rather than runtime behavior changes.

Overview
CI now installs Python dependencies with uv instead of pip by adding a SHA-pinned astral-sh/setup-uv step and switching the install command to uv pip install --system -r requirements-dev.lock for both Python 3.11/3.12 matrix runs.

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