Add repository-scoped Copilot coding instructions

[Copilot]

This PR sets up GitHub Copilot Coding Agent guidance for this repository using the recommended repo-level instruction file. It encodes Shoppy Max–specific constraints so Copilot-generated changes follow existing workflow, safety, and architecture boundaries.

• What was added

  • Created .github/copilot-instructions.md as the canonical Copilot instruction surface for this repo.

• Instruction scope captured

  • Required discovery/read order (AGENTS.md, README.md, routing + relevant controllers/services).
  • Domain invariants to preserve (purchase/GRN/inventory-unit/order/reseller/courier settlement behaviors).
  • Mandatory reuse of existing high-risk service entry points:
    • app/Services/InventoryUnitService.php
    • app/Services/CourierPaymentOrderService.php
  • Cross-DB compatibility guardrails (SQLite + MySQL).
  • Minimal-change expectation (no unrelated refactors).

• Operational guardrails encoded

  • Validation commands to run for relevant changes (php artisan test, npm run build, ./vendor/bin/pint).
  • Explicit safety constraints (no secrets, no .env behavior changes, no ad-hoc SQL bypassing service/controller logic, no sensitive server_utils/ edits unless required).

## 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.

Original prompt

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This section details on the original issue you should resolve

<issue_title>setup copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes setup copilot instructions #11

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