Keep what a consumer gives up on until a person delivers it again or drops it

Design 25 promised a dead-letter stream that did not exist: a message a
consumer gave up on stayed only in its source, which drops it after a week,
and its condition cleared when the advisories stopped (hq issue 330, ADR 0264).

cmd/mesh-controller/seatverbs.go

@@ -250,6 +250,8 @@ func (a *verbArguments) commandLine() ([]string, error) {
 		return argv, nil
 	case "tools":
 		return nil, errors.New("tools is answered from the records, not by a command")
+	case "dead-letters":
+		return nil, errors.New("dead-letters is answered by the serving controller, on its own connection, not by a command")
 	case "status":
 		return []string{"status", "--json"}, nil
 	case "nodes":
@@ -982,6 +984,9 @@ func seatToolHandlers() (map[string]link.ToolHandler, []string, error) {
 				if verb == "calls" {
 					return callsAnswer(link.Calls, a.given["call"])
 				}
+				if verb == "dead-letters" {
+					return deadLettersAnswer(ctx, a)
+				}
 				if verb == "doctor" {
 					// From the serving controller, which runs the self-check and hears the signals
 					// (novox/hq to-be 45 §4): the last verdict at once, or a run now.
@@ -1064,7 +1069,10 @@ func actsOnAPlan(args map[string]any) bool {
 
 // inProcess are the verbs answered by this process rather than by a command it runs: `tools` from
 // the records, `calls` from what this process served.
-var inProcess = map[string]bool{"tools": true, "calls": true, "doctor": true}
+var inProcess = map[string]bool{"tools": true, "calls": true, "doctor": true,
+	// What a consumer gave up on, read and changed on the serving controller's own connection (novox/hq
+	// issue 330).
+	"dead-letters": true}
 
 // answersFirst is a command line whose caller is answered before it runs: a push, by its verb or
 // through `command`. A push sends the machine holding the bus first when its user list changed, the