Implement REUSE-WP-0013 registry establish, update, and stats

Add stats, establish (scaffold, publish-check, discover), and update CLI
commands with optional llm-connect bridge, validate --root for sibling repos,
pytest coverage, and documentation for sibling registry onboarding.

tests/test_llm_bridge.py

@@ -0,0 +1,53 @@
+from __future__ import annotations
+
+import json
+from unittest.mock import patch
+
+import pytest
+
+from reuse_surface.llm_bridge import (
+    extract_json_object,
+    llm_connect_url,
+    request_registry_draft,
+)
+
+
+def test_extract_json_object_from_fenced_block():
+    data = extract_json_object('```json\n{"capabilities": []}\n```')
+    assert data == {"capabilities": []}
+
+
+def test_llm_connect_url_missing_raises():
+    with pytest.raises(ValueError, match="LLM_CONNECT_URL"):
+        llm_connect_url(None)
+
+
+def test_request_registry_draft_mock_http():
+    payload = {
+        "content": json.dumps(
+            {
+                "capabilities": [
+                    {
+                        "id": "capability.demo.sample",
+                        "name": "Sample",
+                        "summary": "Demo capability",
+                    }
+                ]
+            }
+        )
+    }
+
+    class FakeResponse:
+        def __enter__(self):
+            return self
+
+        def __exit__(self, *args):
+            return False
+
+        def read(self):
+            return json.dumps(payload).encode("utf-8")
+
+    with patch.dict("os.environ", {"LLM_CONNECT_URL": "http://llm.test"}):
+        with patch("urllib.request.urlopen", return_value=FakeResponse()):
+            draft = request_registry_draft("test prompt")
+    assert draft["capabilities"][0]["id"] == "capability.demo.sample"