Two non-blocking points from the subvol-upgrade review:
- The start request was `?`-propagated before the migration result was
surfaced, so a restart-side failure (incl. the IPC call itself
erroring) could shadow whether the migration succeeded or failed.
Capture the start result instead and surface the migration outcome
first; the restart-failure messages now point at `hivectl start
--agent <name>` for manual recovery.
- The crash-window case (host dies between the two swap renames, leaving
the agent root missing but the original data under `.<name>.old`) now
detects the leftover and tells the operator to `mv` it back, instead
of a bare "no state dir to upgrade — nothing to do".
cac4a4d65a