docs: add contributor guide

[ayobamiseun]

Summary

This PR adds a contributor guide for the Claude Code Codex plugin and links it from the README so new contributors can find the development workflow without having to infer it from the repo structure.

The new CONTRIBUTING.md covers:

• local setup requirements, including Node.js and the optional global Codex CLI dependency used by the build
• the main repository layout across commands, agents, skills, runtime scripts, hooks, tests, and release tooling
• common development commands for tests, build/type checks, version checks, and version bumps
• testing guidance for where to add coverage based on the area being changed
• generated app-server type notes, including the codex app-server generate-ts command used by prebuild
• release metadata guidance so contributors update package and plugin manifests consistently
• a small PR checklist for docs/runtime contributors

The README now includes a short Contributing section that points readers to the new guide.

Motivation

The existing README is strong for plugin users, but it does not explain how to work on the repository itself. A first-time contributor currently has to inspect package.json, CI, tests, and plugin metadata to understand the local workflow. This guide makes that path more direct and calls out the generated type/build behavior that can otherwise be surprising.

Verification

• npm run check-version

I did not rerun the full test suite for this PR because the change is documentation-only.