Add Copilot instructions (#366)

Author: Copilot

.github/copilot-instructions.md

@@ -0,0 +1,16 @@
+This repository contains the Durable Workflow package for Laravel. It is a Composer library that implements durable, long-running workflows and activities in PHP.
+
+When making changes in this repository:
+
+- Keep changes focused and avoid unrelated refactors.
+- Follow the existing PHP style: add `declare(strict_types=1);` to PHP files and keep code PSR-12 compatible.
+- Preserve the existing architecture around `Workflow`, `Activity`, workflow stubs, signals, and generator-based workflow execution.
+- Add or update PHPUnit tests when behavior changes. Keep unit tests in `tests/Unit` and integration-style coverage in `tests/Feature`.
+- Validate changes with the project quality cycle from the repository root:
+  1. `composer ecs`
+  2. `composer stan`
+  3. `composer unit`
+  4. `composer coverage`
+  5. `composer feature`
+- Treat `composer coverage` as a hard requirement: unit-test coverage must remain at 100%.
+- Prefer solutions that stay compatible with the supported Laravel versions declared in `composer.json`.