Add AGENTS.md with Cursor Cloud development instructions

Co-authored-by: LordSchmackes <LordSchmackes@users.noreply.github.com>

AGENTS.md

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+# FlixCooks Development Reference
+
+## Cursor Cloud specific instructions
+
+### Overview
+FlixCooks is a vanilla PHP recipe web application with a flat-file JSON database. There are **no package managers** (no `composer.json`, no `package.json`), no build step, and no formal test framework.
+
+### Running the development server
+```bash
+php -S localhost:8000
+```
+Open http://localhost:8000 in a browser. The app serves all pages from the project root.
+
+### Linting
+The only available lint is PHP syntax checking:
+```bash
+find /workspace -name "*.php" -exec php -l {} \;
+```
+There are no dedicated linting tools (phpcs, phpstan, etc.) configured.
+
+### Testing
+There is no automated test suite. Validate changes by:
+1. Running `php -l` on modified files to check syntax.
+2. Starting the dev server and manually verifying pages render correctly via `curl` or browser.
+
+### Environment configuration
+- Copy `.env.example` to `.env` before running. The app gracefully handles missing/placeholder Firebase credentials — core recipe browsing works without valid Firebase keys.
+- Firebase Authentication and Firestore are cloud-optional services used only for login, bookmarks, and newsletter features.
+
+### Key caveats
+- The data layer is a flat JSON file at `data/recipes.json` — no database service is needed.
+- The `.htaccess` file is only relevant when running under Apache; the PHP built-in server ignores it.
+- All frontend JS libraries (GSAP, Lenis) are loaded from CDNs — no npm/yarn install is required.