Add local coverage tooling

[dbroeglin]

Why

The repository already has pytest and CI quality checks, but it did not have a local-first way to analyze test coverage. This adds lightweight coverage reporting that stays in developer workflows and GitHub Actions without introducing a hosted coverage service.

Approach

• Documents ADR-0018 choosing pytest-cov over coverage.py for local coverage analysis.
• Adds pytest-cov to the dev dependency group and configures branch coverage scoped to copilot_experiments.
• Updates CI to run pytest with coverage while keeping the pre-push hook on plain pytest for speed.
• Documents the explicit local coverage command and ignores generated coverage artifacts.

There is intentionally no initial coverage threshold. The first run establishes a baseline, currently 73% total coverage, before deciding whether a gate is useful.

Validation

• uv run ruff check --fix .
• uv run ruff format .
• uv run ruff check .
• uv run pytest --cov=copilot_experiments --cov-report=term-missing:skip-covered