Add repository-scoped Copilot coding instructions

.github/copilot-instructions.md

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+# Copilot coding instructions for Shoppy Max
+
+Before making changes, read:
+
+- `AGENTS.md` (operator-level workflow and invariants)
+- `README.md` (user-facing behavior and setup)
+- `routes/web.php` plus the relevant controller/service entry points for the module you touch
+
+## Project-specific requirements
+
+- Preserve business invariants for purchases, GRN, inventory units, orders, reseller flows, and courier settlements.
+- Reuse existing service-layer entry points for high-risk transitions:
+  - `app/Services/InventoryUnitService.php`
+  - `app/Services/CourierPaymentOrderService.php`
+- Keep SQL/query behavior compatible with both SQLite and MySQL.
+- Make the smallest possible focused change; do not refactor unrelated areas.
+
+## Validation expectations
+
+- Run existing checks relevant to your change:
+  - `php artisan test`
+  - `npm run build`
+  - `./vendor/bin/pint`
+- If workflow/status logic is changed, verify the full affected flow (not syntax only), following `AGENTS.md`.
+
+## Safety
+
+- Do not commit secrets or edit `.env` for permanent behavior changes.
+- Do not bypass existing controller/service business rules with ad-hoc SQL status updates.
+- Do not modify sensitive `server_utils/` files unless explicitly required.