Add make check-install for umbrella layout consistency

Verify sibling citation-engine checkout, link dependency resolution,
toolchain, and absence of stale in-repo engine copies. Prints actionable
fix hints when the installation layout is wrong.

Makefile

@@ -1,4 +1,4 @@
-.PHONY: preview dev build test typecheck lint
+.PHONY: preview dev build test typecheck lint check-install
 
 # Build the production bundle then serve it locally with vite preview.
 # Prints the URL (default http://localhost:4173/) once the server is ready.
@@ -21,3 +21,7 @@ typecheck:
 
 lint:
 	pnpm lint
+
+# Verify sibling citation-engine checkout, link dependency, and toolchain.
+check-install:
+	@bash scripts/check-install.sh