Codex support: Update README.md and define AGENTS.md

AGENTS.md

@@ -0,0 +1,7 @@
+# Agent Guidelines
+
+- Run `markdownlint` on any edited Markdown files using the repository configuration with `markdownlint --config .markdownlint.json README.md`. Replace `README.md` with each file you change.
+- If lint errors occur, fix them before creating a pull request.
+- Verify the site builds with `bundle exec jekyll build` when possible.
+- Please name your branch to include codex or agent in the branch name
+- If you cannot run markdownlint please warn your user to setup their environment using the instructions in README.md and document your failure in your pr description

README.md

@@ -112,6 +112,26 @@ This means an adaptor is not available to download from Prebid.org as soon as th
 
 * [jekyll - check for non empty](https://michaelcurrin.github.io/dev-cheatsheets/cheatsheets/jekyll/liquid/conditionals/non-empty.html)
 
+## Codex
+
+To get started with Codex with Ruby 3.4.4 preinstalled on this repo, you can set up your environment like this
+
+```bash
+echo "3.4.4" > .ruby-version
+export NOKOGIRI_USE_SYSTEM_LIBRARIES=1
+gem install jekyll bundler
+npm install boostrap
+gem install github-pages --no-document --force
+npm install -g markdownlint-cli
+bundle install
+```
+
+Codex can then check for linting success with
+
+```bash
+markdownlint --config .markdownlint.json "**/*.md"
+```
+
 ## Thanks
 
 Many thanks to the following people who have submitted content to Prebid.org.  We really appreciate the help!