Answer what the records keep, collect on a person's word, and name a machine's images (hq ADR 0251)

The store's tools and a machine's image pruning decide from the records, so
the controller says them: artifacts (every recorded artifact, kept and why,
eligible, collected on request), collect (the after-build sweep on demand,
a dry run unless confirmed with a why, recorded as a hand act) and images
(what a machine's declaration names, now and as last sent). The sweep is one
implementation with two sets of bounds.

cmd/mesh-controller/handacts.go

@@ -79,6 +79,10 @@ var handActVerbs = []handActVerb{
 	{Verb: "retire approve", Decision: "nothing is retired past its bound without a person (ADR 0230)"},
 	{Verb: "retire reject", Decision: "keeping a consumer active is a person's word (ADR 0230)"},
 	{Verb: "cleanup delete", Decision: "nothing retired is deleted without a person (ADR 0230)"},
+	// The sweep run on a person's word rather than after a build: the same decision the records make, at
+	// a moment the person chose (ADR 0251) — never a repair.
+	{Verb: "collect", Decision: "letting the store go of what the records keep for no reason, now rather " +
+		"than at the next build, is a person's word (ADR 0251)"},
 	{Verb: "bus upgrade", Decision: "the bus is never rolled by the mesh: replacing it is a planned step a " +
 		"person starts (ADR 0236)"},
 	{Verb: "upgrade release-backlog", Decision: "after a release plan failed, the next opens only when a " +