M5c-1 plan: task 04, bxctl chat --speaker NAME, opt-in, no default

The speaker goes on every turn of the chat: interactive, --say and --say's retry. Without the
flag it is None. A name that is empty or holds whitespace or a control character (Unicode), or a
second --speaker, is a usage error: the usage text on stderr and exit status 2. The usage text
gains the flag and one line explaining it. Given: chat_speaker.rs (new) and new versions of
chat.rs, chat_approvals.rs and cli.rs. Task 01 now copies chat.rs-task01, the older chat.rs.

Checks: every before block found once at ref 03; replayed from the task text on a fresh ref-03
worktree with its own build directory, byte-identical to the reference, gate ok with 1158 tests
(1148 + 10); /tmp unchanged around the reference's and the replay's gate runs; 20 of 20
mutations caught; the new tests passed 40 of 40 runs under CPU load; given files pass fmt and
the 500-line check; no chat-template tag in the plan or the given files.

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

docs/plans/M5c1/files/crates/bxctl/tests/chat.rs

@@ -161,6 +161,7 @@ fn run_turn_delivers_every_event_in_order_then_the_answer() {
         &id("s1"),
         "what time is it?",
         false,
+        None,
         &mut |e| seen.push(e.clone()),
     )
     .unwrap();
@@ -189,7 +190,7 @@ fn an_error_frame_is_refused_with_its_code_and_detail() {
         End::Error(ErrorCode::SessionFull, "this conversation is full"),
     );
     let mut seen = 0;
-    let err = run_turn(&fake.socket, &id("s1"), "x", true, &mut |_| seen += 1).unwrap_err();
+    let err = run_turn(&fake.socket, &id("s1"), "x", true, None, &mut |_| seen += 1).unwrap_err();
     assert_eq!(seen, 1, "events before the error are still delivered");
     match err {
         ChatError::Refused(w) => {
@@ -199,7 +200,7 @@ fn an_error_frame_is_refused_with_its_code_and_detail() {
         other => panic!("{other:?}"),
     }
     let e: Box<dyn std::error::Error> =
-        Box::new(run_turn(&fake.socket, &id("s1"), "x", true, &mut |_| {}).unwrap_err());
+        Box::new(run_turn(&fake.socket, &id("s1"), "x", true, None, &mut |_| {}).unwrap_err());
     assert!(e.to_string().contains("session full"), "{e}");
 }
 
@@ -207,7 +208,7 @@ fn an_error_frame_is_refused_with_its_code_and_detail() {
 fn no_loopd_is_a_connect_error() {
     let missing = std::env::temp_dir().join("bxctl-no-such-loopd.sock");
     assert!(matches!(
-        run_turn(&missing, &id("s1"), "x", false, &mut |_| {}),
+        run_turn(&missing, &id("s1"), "x", false, None, &mut |_| {}),
         Err(ChatError::Connect(_))
     ));
 }