Copy node.sent_builds onto the assignments' keys once, and keep the column; list the new store tests (issues 1109, 1096)

Delivery 1 wrote no commit on the key of a pulled-in module, so after the upgrade those modules fell out of SentBuilds. At the start of the lease holder a copy fills every key that names no commit (creating a standing pulled-in key where none stands), overwrites nothing, empties each column it copied, and raises an urgent condition while it fails. Migration 0105 no longer drops node.sent_builds (issue 1112 does).

internal/inventory/migrations/0105-the-chain-leaves-the-store.sql

@@ -47,6 +47,6 @@ drop table pending_assignment;
 -- Its triggers go with it; the function they called goes here. The counter table stays: the controller raises it.
 drop table assignment;
 drop function raise_assignment_generation();
-alter table node drop column sent_builds;
--- No reader of it remains. build_gate stays (issue 1097).
+-- node.sent_builds STAYS, only as the source of the one-time copy onto the assignments' keys (issue 1109); dropping it is
+-- issue 1112. build_gate stays (issue 1097).
 drop table chain_moved;