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.

pyproject.toml

@@ -20,6 +20,9 @@ dev = [
   "httpx>=0.27",
   "pytest>=8.0",
 ]
+llm = [
+  "llm-connect",
+]
 
 [project.scripts]
 reuse-surface = "reuse_surface.cli:main"