feat(postcodes/GF): add French Guiana major-commune postcodes (#1039) (#1421)

Adds 3 well-known commune codes:
  97300 Cayenne (capital)
  97310 Kourou (Centre Spatial Guyanais)
  97320 Saint-Laurent-du-Maroni

GF has no state-level subdivisions in this dataset, so postcodes are
country-only. ~22 communes total in the 973## range; shipping the 3
largest as a high-confidence starter set.

Refs: #1039

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dr5hn

contributions/postcodes/GF.json

@@ -0,0 +1,26 @@
+[
+  {
+    "code": "97300",
+    "country_id": 76,
+    "country_code": "GF",
+    "locality_name": "Cayenne",
+    "type": "full",
+    "source": "manual"
+  },
+  {
+    "code": "97310",
+    "country_id": 76,
+    "country_code": "GF",
+    "locality_name": "Kourou",
+    "type": "full",
+    "source": "manual"
+  },
+  {
+    "code": "97320",
+    "country_id": 76,
+    "country_code": "GF",
+    "locality_name": "Saint-Laurent-du-Maroni",
+    "type": "full",
+    "source": "manual"
+  }
+]