insert/move were top-level only; a block could never move into/out of a
section. content/move-into (relocate to a section child at index, tree-wide)
+ content/promote (lift nested block to top level, subtree intact). Pure
tree transforms like the rest of move.sx; cycle-safe (rejects moving a block
into its own descendant). +13 tests.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
