Record repository UI status in the scanner and tolerate older jsonschema.

Adds the repo-ui-metadata extractor (ui_status yes/candidate/not_detected
plus ui_evidence), falls back to the legacy validator when jsonschema does
not accept a referencing registry, and covers unknown edge endpoints in the
graph explorer.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 7458@bnt-lap001
Assistant-Session: 62534cdf-8348-48a7-9c0d-46e0f74c8eae

tests/test_scanner.py

@@ -104,7 +104,12 @@ def test_scan_repo_emits_schema_valid_deterministic_snapshot(tmp_path: Path) ->
         "readme_title",
         "intent_present",
         "scope_present",
+        "ui_status",
+        "ui_evidence",
     }
+    attributes_by_name = {attribute["name"]: attribute["value"] for attribute in candidates["attributes"]}
+    assert attributes_by_name["ui_status"] == "candidate"
+    assert "package.json" in attributes_by_name["ui_evidence"]
 
     for collection_name in ("nodes", "edges", "attributes"):
         stable_keys = [item["stable_key"] for item in candidates[collection_name]]