Integrate Codecov for CI coverage reporting

[coderabbitai]

Summary

Integrate Codecov into this repository so CI can publish coverage reports and surface coverage information on pull requests.

Context

This work was requested from:

• Parent tracking issue: Add Codecov to all my [public ?] repos idea-garden#8
• Checklist comment: Add Codecov to all my [public ?] repos idea-garden#8 (comment)

Requested by: @couimet

Proposed work

• Confirm this repository's test workflow runs in GitHub Actions.
• Generate a machine-readable coverage report during CI (for example, lcov or Cobertura, depending on the stack).
• Upload the coverage report to Codecov from CI.
• Add any minimal repository configuration needed for Codecov.
• Optionally add a badge or status check once the integration is working.

Acceptance criteria

• A CI workflow in this repository produces a coverage artifact/report.
• Coverage is uploaded successfully to Codecov on the default branch.
• Pull requests show Codecov coverage feedback or status checks.
• Any required setup notes are documented in the repository.

Backlinks

• Add Codecov to all my [public ?] repos idea-garden#8
• Add Codecov to all my [public ?] repos idea-garden#8 (comment)