feat: canonicalize duplicate repo identities

docs/registry-onboarding.md

@@ -81,3 +81,10 @@ repo responsible for the whole external fabric relation model.
 from active State Hub repo records whose local path exists on the current WSL
 host. It registers cross-domain local repos into the Fabric registry while only
 syncing graph snapshots for repos that already have Fabric declarations.
+
+When two active repo records point at the same checkout path, accountability
+root discovery treats them as aliases of one repository identity. The canonical
+identity can be declared with `canonical_slug`; otherwise discovery prefers the
+slug matching the checkout directory name and then the entry with a remote URL.
+Use `split_identity: true` only when shared paths are intentional and should
+remain separate review candidates.