chore: update v1/board.json ahead of monorepo migration

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

Author: 1broseidon

v1/board.json

@@ -193,6 +193,11 @@
             "pattern": "^[a-z][a-z0-9]*-[0-9]+$"
           }
         },
+        "parentId": {
+          "type": "string",
+          "description": "Optional parent task/document ID for first-class parent-child linking",
+          "pattern": "^[a-z][a-z0-9]*-[0-9]+$"
+        },
         "dueDate": {
           "type": "string",
           "description": "Optional due date in ISO 8601 format",