Apply updated Codex repository kit

CONTRIBUTING.md

@@ -0,0 +1,25 @@
+# Contributing
+
+This repository is a Modrinth fork. Read upstream `CLAUDE.md` before changing code.
+
+## Development
+
+Use the existing monorepo commands and package boundaries. Prefer small, focused commits.
+
+```bash
+pnpm install
+pnpm --filter @modrinth/app-frontend run lint
+cargo fmt --check
+cargo clippy --package theseus
+```
+
+## Pull Request Readiness
+
+- Keep the working tree clean.
+- Run the cheapest local checks available.
+- Push and watch Gitea Actions until the workflow succeeds.
+- Document skipped checks when local toolchains are unavailable.
+
+## Security
+
+Do not commit private tokens, credentials, signing keys, or local secrets. Use repository or organization secrets for CI.