M5b plan: task 13, instance lock in brokerd, memoryd and gatewayd

Each locks <home>/run/<role>.lock in main (gatewayd after its secret,
so nothing is made before the secret is read). brokerd's old
second-instance test in serve.rs expected the audit lock's message,
so serve.rs has a new version; task 01 now copies serve.rs-task01.

Checks: given tests fail at ref 12 as the task says (2 each in
brokerd, memoryd, gatewayd; serve.rs passes); the task's quoted edits
applied to ref 12 reproduce ref 13's tree exactly; replay gate ok with
1118 tests (1113 - 1 + 6), /tmp unchanged; 14 mutations, all caught;
the six instance test files stressed 25 runs under 16 busy loops and
20 under 32, no failure.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/plans/M5b/files/crates/brokerd/tests/serve.rs

@@ -216,24 +216,6 @@ fn a_stale_socket_is_replaced() {
     );
 }
 
-#[test]
-fn a_second_brokerd_on_the_same_home_refuses_to_start() {
-    let home = Home::new("serve-twice", 900_000);
-    let _running = home.serve();
-    let second = home.run(&[]);
-    assert_eq!(second.status.code(), Some(1));
-    let text = stderr(&second);
-    assert!(text.contains("brokerd is already running"), "{text}");
-    assert!(
-        text.trim_end()
-            .ends_with("see docs/runbook.md#brokerd-already-running"),
-        "{text}"
-    );
-    // The first still has its sockets.
-    let frames = exchange(&home.admin(), 1, Message::Approvals(Empty {}));
-    assert!(matches!(frames[0].msg, Message::ApprovalList(_)));
-}
-
 #[test]
 fn it_answers_each_socket_and_refuses_the_other_kinds() {
     let home = Home::new("serve-kinds", 900_000);