docs: add contributing guide and dual license (MIT + CC BY 4.0)

CONTRIBUTING.md

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+# Contributing to Codex Decoded
+
+Thanks for your interest in improving **Codex Decoded** — a source-level field guide to
+[`openai/codex`](https://github.com/openai/codex). Corrections, clearer explanations,
+better diagrams, and coverage of new subsystems are all welcome.
+
+This guide covers how to propose changes, the conventions we follow, and how the
+review pipeline works.
+
+## Ground rules
+
+- **Accuracy first.** This is a *source-level* guide. Every claim, code excerpt, and
+  diagram should reflect the actual Codex source. When you add or change technical
+  content, reference the file(s) it comes from in your PR description (e.g.
+  `codex-rs/core/src/tools/registry.rs`).
+- **Match the house style.** Keep prose tight and explanatory; keep code excerpts
+  real and minimal; keep diagrams legible. Look at an existing finished chapter
+  (e.g. `src/chapters/ch5.js`, `ch9.js`, `ch13.js`) as the reference for depth and tone.
+- **Small, focused PRs.** One chapter, fix, or feature per PR is easiest to review.
+
+## Getting set up
+
+```bash
+git clone https://github.com/BenAlaa/codex-decoded.git
+cd codex-decoded
+npm install
+npm run dev      # http://localhost:5173 — live guide with hot reload
+npm run build    # production build into dist/ (must pass before a PR can merge)
+npm run preview  # serve the built dist/ locally
+```
+
+Requires Node 20+.
+
+## Project layout
+
+See the **Project layout** and **How it works** sections in the [README](README.md).
+In short:
+
+- `src/chapters/<id>.js` — one module per page, default-exporting a `() => htmlString`.
+- `src/chapters/index.js` — the `CONTENT` registry mapping chapter id → render function.
+- `src/data/book.js` — table of contents and ordering (`BOOK` / `FLAT`).
+- `src/lib/*` — shared helpers (`code`, `mmd`, `fullPrompt`, graph, map, …).
+- `src/prompts/*.txt` — the verbatim Codex system prompts.
+- `src/styles/*` — design tokens and component styles.
+
+## Making common changes
+
+- **Edit a chapter** — change its module in `src/chapters/`. Compose content with the
+  helpers already imported at the top of the file (`code`, `mmd`, `fullPrompt`, …).
+- **Add a chapter** — create `src/chapters/<id>.js`, register it in
+  `src/chapters/index.js`, and add its entry to `src/data/book.js` so it appears in the
+  table of contents.
+- **Add/replace a prompt** — drop a `.txt` file in `src/prompts/`, import it in
+  `src/lib/prompts.js`, and key it `pr-<name>` so `fullPrompt('pr-<name>', …)` resolves.
+- **Diagrams** — build them with the `mmd()` helper (Mermaid) or the D3/Chart.js
+  helpers in `src/lib/graph.js`. Verify they render in `npm run dev`.
+
+## Commit conventions
+
+We use [Conventional Commits](https://www.conventionalcommits.org/). Examples from the
+existing history:
+
+- `feat(ch14): add chapter 14 — Dispatch lifecycle`
+- `fix(ch16): correct apply_patch grammar excerpt`
+- `docs: expand README with chapter overview`
+- `chore: bump vite`
+
+Common types: `feat`, `fix`, `docs`, `style`, `refactor`, `chore`, `ci`. Use the
+chapter id (`ch5`, `cover`, `map`, …) as the scope for content changes.
+
+## Pull request workflow
+
+`main` is protected — all changes land through pull requests:
+
+1. Branch off `main` (`git switch -c feat/my-change`).
+2. Make your change and confirm **`npm run build` passes** locally.
+3. Push and open a PR against `main`.
+4. **CI** runs a build check on your PR — it must pass.
+5. A maintainer reviews and approves; then the PR is merged.
+6. On merge, the site auto-deploys to
+   [GitHub Pages](https://benalaa.github.io/codex-decoded/).
+
+Direct pushes to `main`, force-pushes, and branch deletion are disabled.
+
+## Licensing of contributions
+
+By contributing, you agree that your contributions are licensed under the same terms
+as the project (see the [License](README.md#license) section):
+
+- **Code** — [MIT](LICENSE).
+- **Book content** (prose, diagrams, images) — [CC BY 4.0](LICENSE-CONTENT).
+
+Only contribute material you have the right to submit under these terms. The verbatim
+Codex prompts reproduced here originate from `openai/codex` (Apache-2.0); don't add
+third-party material that is incompatible with the project's licenses.

LICENSE

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+MIT License
+
+Copyright (c) 2026 Ahmed Alaa
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.