mesh-cli: forward the line's standard input to the controller, with the line alone

A secret given at the controller's terminal through mesh-cli (secret accept … --from -) never reached the
line: the engine forwarded the words only. It now forwards what mesh-cli's standard input held (at most
64 KiB, the SDK's bound) on the same core request to the controller, says it nowhere, and sends none
with a follow of the same call. mesh-sdk moves to f047d0a.

internal/meshcli/meshcli.go

@@ -41,6 +41,10 @@ type Asked struct {
 	// a process another account's session started. Only a login session is a person at a terminal (novox/hq ADR
 	// 0272): the controller makes nothing else the terminal.
 	Session string `json:"session,omitempty"`
+	// Stdin is what mesh-cli's standard input held, at most cli.MaxStdin: carried to the controller with the line
+	// and nowhere else. The engine says it in no journal, keeps it in no state, and never sends it again: a follow
+	// carries none. The controller gives it to a line that runs as its terminal alone.
+	Stdin []byte `json:"stdin,omitempty"`
 }
 
 // Forward asks the controller and follows the line to what came of it (AskTheController): what it printed, that
@@ -279,5 +283,5 @@ func answer(ctx context.Context, conn *net.UnixConn, held *slots, forward Forwar
 	_ = conn.SetDeadline(time.Now().Add(cli.AskWithin))
 	asking, cancel := context.WithTimeout(ctx, cli.AskWithin-30*time.Second)
 	defer cancel()
-	reply(forward(asking, Asked{Line: req.Line, Account: account, UID: uid, Session: session}))
+	reply(forward(asking, Asked{Line: req.Line, Account: account, UID: uid, Session: session, Stdin: req.Stdin}))
 }