[codex] add Responsively App and Checkmate to project list

[Sean-Kenneth-Doherty]

Summary

• Add Responsively App to the suggested projects list.
• Add Checkmate to the suggested projects list.
• Link both entries to their GitHub contribution pages for beginner-friendly issues.
• Include language/domain tags for TypeScript, React, Electron, developer tools, monitoring, infrastructure, responsive design, and good-first-issue discovery.

Contributes to #1.

Validation

• pnpm build
• git diff --check
• Verified both repositories are public, not archived, and currently have open good first issue tickets.
• Verified both avatar URLs return images.

[Sean-Kenneth-Doherty]

Withdrawing to reduce review noise from the large batch of small project-list PRs. I am keeping the more concrete bug/security/accessibility fixes open instead.