Collapse the settings migration from eleven shape-specific walkers into one
key-matched depth-first pass (219 -> 104 lines), and trim the managed-hook
cleanup to its read/strip/write core (120 -> 87 lines).
Fixes two bugs the rewrite exposed: commitMessageAi kept its `enabled` flag
(only automations disable on a retired agent), and already-disabled automations
no longer re-trigger a save.
Co-authored-by: Orca <help@stably.ai>