docs: add contributor guide

CONTRIBUTING.md

@@ -0,0 +1,100 @@
+# Contributing
+
+Thanks for taking the time to improve the Codex plugin for Claude Code. This repository is small, but it has a few moving parts: Claude Code plugin metadata, slash-command prompts, Node.js runtime scripts, generated Codex app-server types, and tests that exercise the whole flow.
+
+## Development Setup
+
+Use Node.js 18.18 or later.
+
+Install dependencies:
+
+```bash
+npm ci
+```
+
+Some build steps use the `codex` binary to generate app-server type definitions. If you plan to run the build locally, install Codex too:
+
+```bash
+npm install -g @openai/codex
+```
+
+You do not need to be logged in to Codex for the unit tests. Runtime behavior that would normally call Codex is covered with fixtures.
+
+## Repository Layout
+
+- `plugins/codex/commands/` contains the Claude Code slash-command definitions.
+- `plugins/codex/agents/` contains the rescue subagent definition.
+- `plugins/codex/skills/` contains helper skills used by the plugin and subagent.
+- `plugins/codex/scripts/` contains the Node.js runtime entrypoints and supporting libraries.
+- `plugins/codex/hooks/` contains Claude Code lifecycle hook configuration.
+- `tests/` contains Node test-runner coverage for command definitions, runtime behavior, rendering, state, git targeting, and process management.
+- `scripts/bump-version.mjs` keeps package and plugin release metadata in sync.
+
+## Common Commands
+
+Run the full test suite:
+
+```bash
+npm test
+```
+
+Run the type/build check:
+
+```bash
+npm run build
+```
+
+Check that version metadata is synchronized across package and plugin manifests:
+
+```bash
+npm run check-version
+```
+
+Bump release metadata:
+
+```bash
+npm run bump-version -- 1.0.7
+```
+
+## Testing Notes
+
+The full test suite intentionally covers background jobs, app-server broker behavior, hook behavior, and mocked Codex runs. It can take a few minutes locally.
+
+When changing runtime behavior, add or update tests near the behavior being changed:
+
+- command prompt/metadata changes usually belong in `tests/commands.test.mjs`
+- review target and diff collection changes usually belong in `tests/git.test.mjs`
+- rendering changes usually belong in `tests/render.test.mjs`
+- state and job lifecycle changes usually belong in `tests/state.test.mjs` or `tests/runtime.test.mjs`
+
+## Generated Types
+
+`npm run build` runs `prebuild`, which creates `plugins/codex/.generated/app-server-types` by calling:
+
+```bash
+codex app-server generate-ts --out plugins/codex/.generated/app-server-types
+```
+
+The checked-in source imports those generated types through `plugins/codex/scripts/lib/app-server-protocol.d.ts`. If the Codex app-server protocol changes, regenerate the types and run the test suite before opening a PR.
+
+## Release Metadata
+
+Version values are stored in multiple places:
+
+- `package.json`
+- `package-lock.json`
+- `plugins/codex/.claude-plugin/plugin.json`
+- `.claude-plugin/marketplace.json`
+
+Use `npm run bump-version -- <version>` rather than editing these by hand, then run `npm run check-version`.
+
+## Pull Request Checklist
+
+Before opening a PR, please run:
+
+```bash
+npm test
+npm run check-version
+```
+
+Also run `npm run build` when you touch app-server integration code, generated types, or files covered by `tsconfig.app-server.json`.

README.md

@@ -19,6 +19,10 @@ they already have.
   - Usage will contribute to your Codex usage limits. [Learn more](https://developers.openai.com/codex/pricing).
 - **Node.js 18.18 or later**
 
+## Contributing
+
+For local development setup, repository layout, test commands, and release metadata notes, see [CONTRIBUTING.md](./CONTRIBUTING.md).
+
 ## Install
 
 Add the marketplace in Claude Code: