mesh-cli: give a terminal line the standard input mesh-cli carried, refuse it to any other, keep it nowhere

A secret given at the controller's terminal through mesh-cli (secret accept … --from -) never reached the
line: every line ran with no standard input. A line that runs as the terminal now reads what mesh-cli
carried (at most 64 KiB); an ordinary call carrying any is refused and nothing runs; the calls record keeps
only that some was given, the journal and the answer nothing of it.

cmd/mesh-controller/meshcli.go

@@ -141,6 +141,12 @@ func runForMeshCLI(ctx context.Context, node string, asked link.CLIAsked, v cliV
 	if v.refused != "" {
 		return link.CLIRefusal(v.refused)
 	}
+	// Standard input is the terminal's alone: a secret given at the terminal reaches `secret accept`, and no ordinary
+	// call is handed what the asker's standard input held (novox/hq ADR 0259 §10, ADR 0272).
+	if len(asked.Stdin) > 0 && !v.terminal {
+		return link.CLIAnswer{Exit: 1, Why: v.why, Refused: "standard input is given to a line that runs as the " +
+			"controller's terminal alone, and this one does not. Nothing ran"}
+	}
 	if cliServers[asked.Line[0]] {
 		return link.CLIAnswer{Exit: 1, Why: v.why, Refused: fmt.Sprintf("%s serves until stopped, and is not a "+
 			"command line mesh-cli runs. Nothing ran", asked.Line[0])}
@@ -155,8 +161,12 @@ func runForMeshCLI(ctx context.Context, node string, asked link.CLIAsked, v cliV
 	}
 	cmd := selfCommand(ctx, line)
 	cmd.Env = commandEnvironment(fmt.Sprintf("%s through mesh-cli on %s", asked.Account, node), verb, v.terminal)
-	// No standard input: a command that reads one gets nothing, and fails saying so (ADR 0272 §5).
+	// No standard input unless mesh-cli carried one for a terminal line: a command that reads one gets nothing, and
+	// fails saying so (ADR 0272 §5).
 	cmd.Stdin = nil
+	if len(asked.Stdin) > 0 {
+		cmd.Stdin = bytes.NewReader(asked.Stdin)
+	}
 	var stdout, stderr bytes.Buffer
 	cmd.Stdout, cmd.Stderr = &stdout, &stderr
 	err := cmd.Run()