Add AGENTS.md with coding guidelines for AI agents

AGENTS.md

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+# Agent Instructions
+
+This document provides guidance for AI agents working on the Open Source Point of Sale (OSPOS) codebase.
+
+## Code Style
+
+- Follow PHP CodeIgniter 4 coding standards
+- Run PHP-CS-Fixer before committing: `vendor/bin/php-cs-fixer fix --config=.php-cs-fixer.no-header.php`
+- Write PHP 8.1+ compatible code with proper type declarations
+- Use PSR-12 naming conventions: `camelCase` for variables and functions, `PascalCase` for classes, `UPPER_CASE` for constants
+
+## Development
+
+- Create a new git worktree for each issue, based on the latest state of `origin/master`
+- Commit fixes to the worktree and push to the remote
+
+## Testing
+
+- Run PHPUnit tests: `composer test`
+- Tests must pass before submitting changes
+
+## Build
+
+- Install dependencies: `composer install && npm install`
+- Build assets: `npm run build` or `gulp`
+
+## Conventions
+
+- Controllers go in `app/Controllers/`
+- Models go in `app/Models/`
+- Views go in `app/Views/`
+- Database migrations in `app/Database/Migrations/`
+- Use CodeIgniter 4 framework patterns and helpers
+- Sanitize user input; escape output using `esc()` helper
+
+## Security
+
+- Never commit secrets, credentials, or `.env` files
+- Use parameterized queries to prevent SQL injection
+- Validate and sanitize all user input