Remove an image's pushed names with its build tag, and take no work under the free-space floor (issue 1834)

The registry push tags the image a second time, so removing only the build tag freed nothing.
The held image a comparison pulls goes after it is read, and a node under the floor leaves the ask
to another build node instead of taking it and failing.

cmd/mesh-builder/busy.go

@@ -9,6 +9,8 @@ import (
 	"sync"
 	"sync/atomic"
 	"time"
+
+	"github.com/novox/mesh-controller/internal/builder"
 )
 
 // **The build agent says whether it is busy, and being asked drains it** (novox/hq ADR 0335 §1, §5). Its
@@ -211,3 +213,23 @@ func busy() (string, error) {
 	}
 	return fmt.Sprintf("busy: %s, in hand since %s; taking no new build", what, since), nil
 }
+
+var (
+	roomMu   sync.Mutex
+	roomSaid bool
+)
+
+// noRoom says this node is under the floor of free space (novox/hq issue 1834): it takes no work, so another build node
+// takes the ask instead of this one taking it and ending it in an error. Said once when it starts and again when it ends.
+func noRoom(workspace string) bool {
+	err := builder.RoomToWork("/", workspace)
+	roomMu.Lock()
+	defer roomMu.Unlock()
+	if err != nil && !roomSaid {
+		fmt.Fprintf(os.Stderr, "taking no builds: %v\n", err)
+	} else if err == nil && roomSaid {
+		fmt.Fprintf(os.Stderr, "there is room again: taking builds\n")
+	}
+	roomSaid = err != nil
+	return err != nil
+}