fix(signal): warn instead of silently dropping a failed ctrl-c handler

install_ctrl_c() used `if ctrl_c().await.is_ok()`, so if the handler
failed to install, ctrl-c silently stopped working with no diagnostic.
Match on the Result and log a warning (then bail the task — the second
arm would only fail the same way).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/common/signal.rs

@@ -7,10 +7,17 @@ pub fn install_ctrl_c() -> CancellationToken {
     let token = CancellationToken::new();
     let trigger = token.clone();
     tokio::spawn(async move {
-        if tokio::signal::ctrl_c().await.is_ok() {
-            tracing::info!("ctrl-c received, shutting down");
-            trigger.cancel();
+        if let Err(e) = tokio::signal::ctrl_c().await {
+            // Installing the handler failed — ctrl-c won't trigger a graceful
+            // shutdown. Say so instead of failing silently; the user can still
+            // kill the process, and the second-ctrl-c arm below would only fail
+            // the same way, so bail out of the task.
+            tracing::warn!("could not install ctrl-c handler: {e}; ctrl-c won't shut down cleanly");
+            return;
         }
+        tracing::info!("ctrl-c received, shutting down");
+        trigger.cancel();
+
         if tokio::signal::ctrl_c().await.is_ok() {
             tracing::warn!("second ctrl-c — exiting now");
             std::process::exit(130);