Create run workspaces only when needed

docs/internal/workspace.md

@@ -45,6 +45,11 @@ file retains owner metadata after release or process death; its existence is
 not evidence that a lock is active. Command and restore flows wait for this
 lock only while their context remains active, and report a release failure.
 
+The runner assigns every invocation a run-workspace identity but creates the
+physical directory only before an executing stage that requires it. Fully
+reused invocations and cleanup-only invocations therefore do not accumulate
+empty run directories.
+
 ## Cleanup Semantics
 
 The canonical cleanup stage: