Implement REUSE-WP-0012 federation scale and intent alignment

Add hub sync and report cohorts CLI commands with pytest coverage, document
sibling index publish contract and hub hardening path, align INTENT layout,
raise external evidence on three registry entries, and close gap priorities
19-23 (priority 18 deferred on sibling index blocks).

.gitea/workflows/ci.yml

@@ -32,5 +32,8 @@ jobs:
           reuse-surface catalog
           reuse-surface graph --check --fail-on-warnings
 
+      - name: Planning cohort report (informational)
+        run: reuse-surface report cohorts --planning-min D4 || true
+
       - name: Run tests
         run: pytest -q