Add Code Coverage Infrastructure

[lmcrean]

Adds code coverage reporting infrastructure to docorator using covr and Codecov integration. This establishes the foundation for tracking test coverage metrics without modifying any package functionality.

Changes

• DESCRIPTION: Added covr to Suggests
• README.md: Added Codecov badge
• .github/workflows/test-coverage.yml: New GitHub Actions workflow for coverage reporting

Testing for Reviewers

# Install package with suggests
devtools::install(dependencies = TRUE)

# Verify covr can be loaded
library(covr)

# Run coverage (will work locally)
package_coverage()

Prerequisites for Full Functionality

Repository admin must enable Codecov:

1. Visit https://github.com/apps/codecov
2. Grant access to GSK-Biostatistics/docorator

Without this, the workflow will run but fail at the upload step (expected).

Notes

• Workflow triggers on PRs and pushes to main/master
• Uses standard r-lib/actions templates
• No breaking changes -- safe to implement
• Coverage percentages will be available after Codecov activation

addresses #16