docs: add code conventions

[britle20]

Summary

Closes #8.

Adds repository-wide code conventions so future agent-authored PRs follow consistent naming, file organization, TypeScript, React, CSS Modules, design token, domain naming, test, and comment rules.

Changes

• Added docs/code-conventions.md.
• Linked the conventions document from AGENTS.md.
• Added the conventions document to the README documentation map.
• Merged latest develop after scaffold PR feat: scaffold Vite React app #7 landed.
• Updated PLANS.md Active Issue Order to remove completed scaffold work and mark Document code conventions #8 / PR docs: add code conventions #9 as in review.
• Updated docs/features/01-project-scaffold.md status to Complete.

Testing

• git diff --check
• Markdown internal reference check
• npm run typecheck
• npm run lint
• npm run test
• npm run build

Notes / Limitations

• Docs-focused change; no production app behavior changed.
• Issue Project Scaffold #6 was closed manually after PR feat: scaffold Vite React app #7 merged because GitHub did not auto-close it from the develop-targeted PR.