feat(cli): add command-line interface

examples/invoice.json

@@ -0,0 +1,24 @@
+{
+  "number": "FV/2026/08/001",
+  "issueDate": "2026-08-28",
+  "dueDate": "2026-09-11",
+  "currency": "PLN",
+  "seller": {
+    "name": "Vulp Sp. z o.o.",
+    "address": "ul. Przykladowa 1, 00-001 Warszawa",
+    "taxId": "PL1234567890",
+    "email": "billing@vulp.example"
+  },
+  "buyer": {
+    "name": "Acme Corp",
+    "address": "ul. Klienta 5, 30-001 Krakow",
+    "taxId": "PL9876543210"
+  },
+  "paymentTerms": "14 days, transfer",
+  "notes": "Thank you for your business.",
+  "lines": [
+    {"description": "Consulting hours", "quantity": 12, "unitPrice": 150.0, "vat": 23, "unit": "h"},
+    {"description": "Annual software license", "quantity": 2, "unitPrice": 500.0, "vat": 23, "discount": 0.1},
+    {"description": "Printed manual", "quantity": 4, "unitPrice": 25.0, "vat": 5, "unit": "pcs"}
+  ]
+}