ci: add pytest-cov coverage reporting

## Problem

No test coverage reporting exists. Cannot quantify test gaps or track improvement.

## Scope

1. Add `pytest-cov` to CI workflows
2. Upload coverage to Codecov (or similar)
3. Add coverage badge to README
4. Establish baseline coverage percentage

## Acceptance Criteria
- [ ] Coverage report generated in CI
- [ ] Coverage badge in README
- [ ] Baseline coverage documented