feat(okf): add okf_validate; harden YAML parser for block lists + folded scalars

Verified conformant against Google reference bundles (crypto_bitcoin, ga4, stackoverflow).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

custom-skills/97-ourdigital-okf/code/scripts/tests/test_okf_common.py

@@ -24,6 +24,21 @@ class TestFrontmatter(unittest.TestCase):
     def test_empty_list(self):
         self.assertEqual(parse_yaml_subset("tags: []")["tags"], [])
 
+    def test_block_list(self):
+        raw = "type: BigQuery Dataset\ntags:\n- cryptocurrency\n- bitcoin\n- public data\ntimestamp: '2026-05-28T22:44:47+00:00'"
+        meta = parse_yaml_subset(raw)
+        self.assertEqual(meta["tags"], ["cryptocurrency", "bitcoin", "public data"])
+        self.assertEqual(meta["timestamp"], "2026-05-28T22:44:47+00:00")
+
+    def test_folded_multiline_scalar(self):
+        raw = "description: This dataset contains a complete history of the Bitcoin\n  blockchain and updates every 10 minutes.\ntype: BigQuery Dataset"
+        meta = parse_yaml_subset(raw)
+        self.assertEqual(
+            meta["description"],
+            "This dataset contains a complete history of the Bitcoin blockchain and updates every 10 minutes.",
+        )
+        self.assertEqual(meta["type"], "BigQuery Dataset")
+
     def test_no_frontmatter(self):
         meta, body = parse_frontmatter("# Just markdown\n")
         self.assertIsNone(meta)