fix: add 'reach' option comment layout for codecov configuration

[reddevilmidzy]

♟️ What’s this PR about?

PR에 codecov가 코멘트를 달아주지만 전체 커버리지를 보여주지 않아 해당 설정을 추가하였습니다.

🔗 Related Issues / PRs

part of #167

[Copilot]

Pull Request Overview

This PR updates the Codecov comment configuration to include branch reach information and enable carryforward flags in coverage comments.

• Adds the reach token to the comment layout so reach coverage is displayed.
• Enables show_carryforward_flags to surface any carryforward flags in the comment.

Comments suppressed due to low confidence (2)

.github/codecov.yml:26

• [nitpick] Consider converting the comma-separated layout string into a YAML sequence (e.g., `layout:
  • header
  • reach
  • diff
    ...`) to improve readability and ease future updates.

  layout: "header, reach, diff, flags, files, footer"

.github/codecov.yml:32

• [nitpick] Add a comment explaining what show_carryforward_flags does and when it should be used, so future maintainers understand its effect on the Codecov comment output.

  show_carryforward_flags: true