ci: add pytest-cov coverage reporting

[dtsong]

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