Bundles the MUL-3404 disk-usage feature with the two Preflight BLOCK fixes.
- feat(daemon): `disk-usage --all-profiles` aggregates across every workspace
root (default + each ~/.multica/profiles/* root, incl. the Desktop app's),
with a per-root breakdown and combined grand total; the cross-root hint now
also fires when the current root is non-empty.
- fix(db): drop DB-level foreign keys/cascades from the new autopilot_subscriber
and comment.source_task_id migrations (resolved in the app layer — autopilot
delete now removes subscribers in a transaction); the autopilot_subscriber
down-migration relabels reason='autopilot' to 'manual' instead of deleting.
- fix(server): readiness verifies every required migration is applied, not just
the lexically-last one, so an out-of-order migration can't be masked.
MUL-3404.
