006a T2: Stack merges under `gt`: mid-stack extraction, safe restack nf-3au ← Beads

Twinned from https://github.com/cfsh/ninefold/issues/943 by tools/beads/import_github.py's reconcile pass.

From 006a-the-oracle-and-the-remaining-verbs.md's task table (design 006a). - Depends on: T1 - Covers: 20 (narrowed)