Add AGENTS.md and AI contribution guidance

.github/PULL_REQUEST_TEMPLATE.md

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 <!-- Please describe your changes. -->
 
+## Use of AI Tools
+
+<!-- Please disclose whether AI tooling was used to author the code in the pull request. To what extent was AI used? Was it used as a code reviewer, or was it used to "vibe code" everything? It is your responsibility to review and take responsibility for what AI generates. For more, see CONTRIBUTING.md which includes instructions for how to provide instructions to AI agents. -->
 
 
 <!--

AGENTS.md

@@ -0,0 +1,134 @@
+# Performance Lab
+
+This is a monorepo for the WordPress Performance Team, containing a collection of standalone performance feature plugins. Refer to the [Performance Lab handbook](https://make.wordpress.org/performance/handbook/performance-lab/) for more details. 
+
+## Project Overview
+
+* **Purpose:** To develop and maintain a suite of plugins that improve the performance of WordPress sites. All should be considered 
+* **Technologies:** PHP, JavaScript, CSS, a variety of testing and linting tools.
+* **Architecture:** This is a monorepo containing multiple WordPress plugins. The main plugin is `performance-lab`, which acts as a hub for the other feature plugins, most of which are standalone plugins located in the `plugins` directory (although some come from other repos). Each plugin has its own tests located in its respective `tests` subdirectory.
+
+## Building and Running
+
+### Prerequisites
+
+* [Node.js and npm](https://nodejs.org/en/)
+* [Docker](https://www.docker.com/)
+* [Git](https://git-scm.com/)
+* [Composer](https://getcomposer.org/)
+
+### Installation
+
+1. Clone the repository, e.g. `git clone https://github.com/WordPress/performance.git`.
+2. Run `npm install` to install the Node.js dependencies.
+3. Run `composer install` to install the PHP dependencies.
+
+### Building
+
+* To build the JavaScript and CSS assets: `npm run build``
+* To build all plugins and place into the `build` directory: `npm run build-plugins`
+* To build a specific plugin: `npm run build:plugin:<plugin-slug>` (e.g., `npm run build:plugin:performance-lab`)
+* To build ZIP files for distribution: `npm run build-plugins:zip`
+
+### Running a Local Environment
+
+This project uses `@wordpress/env` to create a local development environment.
+
+* Check if the environment is already running: `npm run wp-env status`
+* Start the environment: `npm run wp-env start`
+* Stop the environment: `npm run wp-env stop`
+
+The environment will by default be located at `http://localhost:8888` but this can be overridden by `.wp-env.override.json`.
+
+## Code Style
+
+In general, the [coding standards for WordPress](https://developer.wordpress.org/coding-standards/wordpress-coding-standards/) should be followed:
+
+* [CSS Coding Standards](https://developer.wordpress.org/coding-standards/wordpress-coding-standards/css/)
+* [HTML Coding Standards](https://developer.wordpress.org/coding-standards/wordpress-coding-standards/html/)
+* [JavaScript Coding Standards](https://developer.wordpress.org/coding-standards/wordpress-coding-standards/javascript/)
+* [PHP Coding Standards](https://developer.wordpress.org/coding-standards/wordpress-coding-standards/php/)
+
+Note that for the JavaScript Coding Standards, the code should also be formatted using Prettier, specifically the [wp-prettier](https://www.npmjs.com/package/wp-prettier) fork with the `--paren-spacing` option which inserts many extra spaces inside parentheses.
+
+For the HTML Coding Standards, disregard the guidance that void/empty tags should be self-closing, such as `IMG`, `BR`, `LINK`, or `META`. This is only relevant for XML (XHTML), not HTML. So instead of `<br />` this should only use `<br>`, for example.
+
+Additionally, the [inline documentation standards for WordPress](https://developer.wordpress.org/coding-standards/inline-documentation-standards/) should be followed:
+
+* [PHP Documentation Standards](https://developer.wordpress.org/coding-standards/inline-documentation-standards/php/)
+* [JavaScript Documentation Standards](https://developer.wordpress.org/coding-standards/inline-documentation-standards/javascript/)
+
+Note that `lint-staged` will be used to automatically run code quality checks with the tooling based on the staged files.
+
+### Indentation
+
+In general, indentation should use tabs. Refer to `.editorconfig` in the project root for specifics.
+
+### Inline Documentation
+
+It is expected for code introduced in pull requests to have `@since` tags with the `n.e.x.t` placeholder version. It will get replaced with the actual version at the time of release. Do not add any code review comments such code.
+
+Every file, function, class, method constant, and global variable must have an associated docblock with a `@since` tag.
+
+### PHP
+
+Whenever possible, the most specific PHP type hints should be used, when compatible with the minimum version of PHP supported by WordPress, unless the `testVersion` config in `phpcs.xml.dist` is higher.
+
+When native PHP type cannot be used, PHPStan's [PHPDoc Types](https://phpstan.org/writing-php-code/phpdoc-types) should be used, including not only the basic types but also subtypes like `non-empty-string`, [integer ranges](https://phpstan.org/writing-php-code/phpdoc-types#integer-ranges), [general arrays](https://phpstan.org/writing-php-code/phpdoc-types#general-arrays), and especially [array shapes](https://phpstan.org/writing-php-code/phpdoc-types#array-shapes). The types should comply with PHPStan's level 10.
+
+The one exception for using PHP types is whenever a function is used as a filter. Since plugins can supply any value at all when filtering, it is important for the filtered value to always be `mixed`. The first statement in the function in this case must always check the type, and if it is not the expected type, override it to be so. For example:
+
+```php
+/**
+ * Filters foo.
+ *
+ * @param string|mixed $foo Foo.
+ * @return string Foo.
+ */
+function filter_foo( $foo, int $post_id ): string {
+	if ( ! is_string( $foo ) ) {
+		$foo = '';
+	}
+	/**
+	 * Because plugins do bad things.
+	 *
+	 * @var string $foo
+	 */
+
+	 // Filtering logic goes here.
+
+	 return $foo;
+}
+add_filter( 'foo', 'filter_foo', 10, 2 );
+```
+
+All PHP files should have a namespace which coincides with the `@package` tag in the file's PHPDoc header.
+
+### JavaScript
+
+All JavaScript code should be written with JSDoc comments. All function parameters, return values, and other types should use [TypeScript in JSDoc](https://www.typescriptlang.org/docs/handbook/jsdoc-supported-types.html).
+
+JavaScript code is written using ES modules. This JS code must be runnable as-is without having to go through a build step, so it must be plain JavaScript and not TypeScript. The project _may_ also distribute minified versions of these JS files.
+
+Never render HTML `script` markup directly. Always use the relevant APIs in WordPress for adding scripts, including `wp_enqueue_script()`, `wp_add_inline_script()`, `wp_localize_script()`, `wp_print_script_tag()`, `wp_print_inline_script_tag()`, `wp_enqueue_script_module()` among others. Since script modules are used, new scripts should normally have a `type="module"` when printing via `wp_print_inline_script_tag()` and when an external script is used, then `wp_enqueue_script_module()` is preferred.
+
+### Static Analysis Commands
+
+* **PHPStan**: `npm run phpstan`
+* **TypeScript**: `npm run tsc`
+
+### Linting Commands
+
+* **JavaScript:** `npm run lint-js`
+* **PHP:** `npm run lint-php`
+
+### Formatting Commands
+
+* **JavaScript:** `npm run format-js`
+* **PHP:** `npm run format-php`
+
+### Testing Commands
+
+* **End-to-end (E2E) tests:** `npm run test-e2e`
+* **PHP tests:** `npm run test-php`
+* **PHP tests (multisite):** `npm run test-php-multisite`

CONTRIBUTING.md

@@ -9,13 +9,17 @@ In general, all code must follow the [WordPress Coding Standards and best practi
 - **WordPress**: As of Performance Lab v3.3.0, released July 15, 2024, the plugin's minimum WordPress version requirement is 6.5.
 - **PHP**: Always match the latest WordPress version. The minimum required version right now is 7.2.
 
-
 ## Guidelines
 
 - As with all WordPress projects, we want to ensure a welcoming environment for everyone. With that in mind, all contributors are expected to follow our [Code of Conduct](https://make.wordpress.org/handbook/community-code-of-conduct/).
-
 - All WordPress projects are [licensed under the GPLv2+](/LICENSE), and all contributions to Gutenberg will be released under the GPLv2+ license. You maintain copyright over any contribution you make, and by submitting a pull request, you are agreeing to release that contribution under the GPLv2+ license.
 
+## Use of AI Tools
+
+You are free to use artificial intelligence (AI) tooling to contribute, but we ask that you disclose what tooling you are using and to what extent a pull request has been authored by AI. Are you using AI as a code reviewer, or are you using AI to "vibe code" everything? It is your responsibility to review and take responsibility for what AI generates.
+
+This repo includes an [`AGENTS.md`](./AGENTS.md) file which is a [README for agents](https://agents.md/). This can be used by tools Cursor, Gemini CLI, GitHub Copilot coding agent, OpenAI Codex, and others. If your AI tool doesn't support `AGENTS.md` directly, you can simply create a symlink to it using the file name your tool is looking for.
+
 ## Reporting Security Issues
 
 Please see [SECURITY.md](/SECURITY.md).