Add workspace filesystem helpers

docs/internal/overview.md

@@ -29,6 +29,9 @@ belongs in modules, not in command handlers.
   diagnostics artifact writers, atomic writes, and retention decisions.
 - `internal/core/source`: source documents, source units, source references, and
   validation.
+- `internal/core/workspace`: effective workspace roots, enabled-state helpers,
+  safe workspace-relative path construction, and atomic workspace artifact
+  writes.
 
 Core packages should remain deterministic and concrete. They should not import
 production modules.