Ship full split-invoice stock purchase order workflow:
- Add 5 new MCP tools for per-line stock purchase item management and catalogue updates
- Add idempotent database migration adding sku, unit, and supplier_sku fields to stock_purchase_items
- Implement service layers for all new operations, including auto-closing the parent PO when all lines are fully collected
- Add complete test coverage for new services and API routes
- Update all relevant documentation including MCP API specs, agent instructions, and feature request logs