Leave a busy container running until it says idle, within 15 minutes

A send that replaced the build agent killed the check it ran, and the queue
started it again elsewhere (issue 594). A container may now declare busy; the
node-engine asks it before replacing it, applies everything else, says the
wait, and replaces it on the first idle pass or at the bound, kept across
restarts (novox/hq ADR 0335).

internal/store/store.go

@@ -155,6 +155,11 @@ type Applied struct {
 	// container's spec label; this is the same information kept per file, so that when the spec
 	// no longer matches the host can say WHICH file changed rather than only that something did.
 	Reads map[string]string `json:"reads,omitempty"`
+
+	// BusySince is, for a container left running because it said busy, when the first apply that would
+	// have replaced it ran (novox/hq ADR 0335 §3): kept here, beside the machine's state, so the bound runs
+	// from then across every pass and across a restart of the node-engine. Absent while nothing waits.
+	BusySince *time.Time `json:"busy_since,omitempty"`
 }
 
 // Into is what a file written into held before the mesh's keys, or before the mesh's block.