Add AGENTS.md with instructions for AI coding agents

AGENTS.md

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+# AGENTS.md
+
+Instructions for AI agents working on this project.
+
+## Fundamental rule: never run PHP/Node/Composer on the host machine
+
+Everything goes through the `builder` container, via Castor:
+
+```bash
+castor builder -- bin/console cache:clear    # one-off command
+castor builder -- bin/console make:migration # one-off command
+```
+
+Host prerequisites only: Docker, Bash, Castor.
+
+## Essential commands
+
+```bash
+castor start                                 # build + install + up + migrate
+castor stop                                  # stop the stack
+castor logs [--service=service]              # logs (frontend, postgres, ...)
+castor app:install                           # composer install + yarn/npm ci + importmap + qa:install
+castor app:db:migrate                        # Doctrine migrations (alias: castor migrate)
+castor app:db:fixtures                       # fixtures (alias: castor fixtures)
+castor postgres -- select 1 from foobar      # one-off command database query
+```
+
+Docker / workers:
+
+```bash
+castor docker:build [--service=service]
+castor docker:up [--service=service]
+castor start-workers                         # start workers (worker profile)
+castor stop-workers
+```
+
+## Castor contexts
+
+The context changes how tasks are executed (`APP_ENV`, compose files, etc.):
+
+```bash
+castor --context=test qa:phpunit             # APP_ENV=test, for tests
+castor --context=ci ...                      # like test, tuned for CI
+```
+
+Always run tests and anything touching the database with `--context=test`.
+Without option, the `default` context applies.
+
+## Stack
+
+- Symfony in `application/` (docroot = `application/public`)
+- PostgreSQL 16: user/pass/db = `app`/`app`, DATABASE_URL already configured
+- nginx + php-fpm (service `frontend`), Traefik router, HTTPS on `<root_domain>` (see `castor.php`)
+- Node/yarn only inside the `builder` container
+
+## QA — before considering a task done
+
+Tools run inside the builder.
+
+```bash
+castor qa                                    # everything: cs + phpstan + twig-cs + phpunit
+castor qa:cs [--dry-run]                     # PHP-CS-Fixer (.php-cs-fixer.php)
+castor qa:phpstan [-b]                       # PHPStan level 8 (phpstan.neon)
+castor qa:twig-cs                            # Twig-CS-Fixer
+castor qa:phpunit                            # PHPUnit (if present in application/)
+```
+
+After any PHP/Twig code change: `castor qa:cs --dry-run`,
+`castor qa:phpstan`, then `castor qa:phpunit`.
+
+## Conventions
+
+1. **Never invoke `docker compose` by hand**: use the `docker_compose()` /
+   `docker_compose_run()` functions from `.castor/docker.php` to write new tasks.
+2. **Never hardcode ports or project names**: git worktree support automatically
+   isolates project/volumes/ports. Use `variable('project_name')` etc.
+3. New recurring task? Make it a Castor task (`castor.php` or `.castor/*.php`),
+   not a shell script.
+4. QA tool dependencies live in `tools/<tool>/composer.json`
+   (not in `application/composer.json`).