No change to a machine takes effect unseen (hq ADR 0217, to-be 44)

Two incidents had one shape: a change took effect that nobody saw first (hq issues 241, 304).
Three guards, each silent when nothing is at stake:

- settings show [--history], and a set that answers each key it adds, changes and removes, and
  refuses a removal unless --replace; the replaced or cleared layer is kept in settings_history,
  in the same transaction as the write (migration 0078).
- every send keeps a summary of what it sent (no file content), plan <node> --diff compares with
  it, and push with no machine is refused unless --all.
- a push that would give a running container's mount another host directory holds that machine,
  naming the module, mount and both directories, until push <node> --move <module>; a named push's
  cascade is held the same way.

Rebased onto main and fitted to it: the hold runs before the push says what it recreates, a whole
push still says so first and leaves machines waiting for a gate, the verb's push with no machine is
still --behind and a push still needs why. The verbs take plan diff, settings replace and history,
and push move beside a machine, each refused where it cannot take effect.

cmd/mesh-controller/licence.go

@@ -232,7 +232,7 @@ func licenceKey(ctx context.Context, args []string) error {
 	// and printing the value here would put the one copy that matters on a terminal.
 	fmt.Printf("sealed to %d holder(s). The mesh has discarded the key and cannot read it back\n",
 		sealed)
-	fmt.Printf("  run `push` to deliver it\n")
+	fmt.Printf("  run `push --behind` to deliver it\n")
 	return nil
 }
 
@@ -340,7 +340,7 @@ func licenceSetGrant(ctx context.Context, args []string) error {
 		return err
 	}
 	fmt.Printf("%s now holds a refresh token for %s, sealed to that node's key and readable by it "+
-		"alone.\n  the control plane stored the box without opening it; run `push` to deliver it\n",
+		"alone.\n  the control plane stored the box without opening it; run `push --behind` to deliver it\n",
 		"the manager", name)
 	return nil
 }
@@ -423,7 +423,7 @@ func licenceSubmitRefresh(ctx context.Context, args []string) error {
 			"manager node alone"
 	}
 	fmt.Printf("submitted a refresh for %s: a new access token sealed to %d holder(s), and %s.\n"+
-		"  run `push` to deliver it\n", name, sealed, rotatedNote)
+		"  run `push --behind` to deliver it\n", name, sealed, rotatedNote)
 	return nil
 }
 
@@ -456,7 +456,7 @@ func licenceRefresh(ctx context.Context, args []string) error {
 		return err
 	}
 	fmt.Printf("refreshed %s: a new access token sealed to %d holder(s), and the refresh token left "+
-		"with its manager.\n  run `push` to deliver it\n", name, sealed)
+		"with its manager.\n  run `push --behind` to deliver it\n", name, sealed)
 	return nil
 }