fix: mvp-chat cleanup leak

Stop the mvp-chat process on Ctrl-C/SIGTERM so it tears down cleanly instead of leaking past signal delivery.

- orchestrator_app: spawn_stop_listener now spawns a Linux SIGINT/SIGTERM handler (signal_hook) that sends the shutdown signal alongside the existing stdin "stop"/"shutdown"/"quit" listener; on non-Linux the spare sender is dropped.
- orchestrator_app: split the channel sender into a stdin_tx clone so the stdin thread and the signal thread each own a sender without moving it out of scope.

Signed-off-by: Zachery Aaron Shores-Chmielewski <zacheryasc@gmail.com>

crates/mvp-system/src/orchestrator_app.rs

@@ -4660,6 +4660,7 @@ fn stop_requested(stop_rx: &mpsc::Receiver<()>) -> bool {
 
 fn spawn_stop_listener() -> mpsc::Receiver<()> {
     let (tx, rx) = mpsc::channel();
+    let stdin_tx = tx.clone();
     thread::spawn(move || {
         let stdin = std::io::stdin();
         for line in stdin.lock().lines().map_while(Result::ok) {
@@ -4668,11 +4669,29 @@ fn spawn_stop_listener() -> mpsc::Receiver<()> {
                 || trimmed.eq_ignore_ascii_case("shutdown")
                 || trimmed.eq_ignore_ascii_case("quit")
             {
-                let _ = tx.send(());
+                let _ = stdin_tx.send(());
                 break;
             }
         }
     });
+    #[cfg(target_os = "linux")]
+    {
+        thread::spawn(move || {
+            let Ok(mut signals) = signal_hook::iterator::Signals::new([
+                signal_hook::consts::signal::SIGINT,
+                signal_hook::consts::signal::SIGTERM,
+            ]) else {
+                return;
+            };
+            if signals.forever().next().is_some() {
+                let _ = tx.send(());
+            }
+        });
+    }
+    #[cfg(not(target_os = "linux"))]
+    {
+        drop(tx);
+    }
     rx
 }