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_stats.py

@@ -0,0 +1,20 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+from reuse_surface.stats import collect_stats, format_stats_markdown
+
+
+def test_collect_stats_on_repo_root():
+    root = Path(__file__).resolve().parent.parent
+    stats = collect_stats(root)
+    assert stats["capability_count"] == 20
+    assert stats["index_present"] is True
+    assert "discovery" in stats["histograms"]
+
+
+def test_format_stats_markdown_contains_count():
+    root = Path(__file__).resolve().parent.parent
+    text = format_stats_markdown(collect_stats(root))
+    assert "Capabilities:" in text
+    assert "20" in text