fix(daemon): log fatal startup errors to daemon.log

A daemon that fails during startup reported the reason via eprintln! in
main's error arm. Under launchd stderr is discarded, so the failure was
invisible: daemon.log was created and left empty — exactly the case the
log file was added for.

Found by the manual acceptance run: the launchd agent loaded correctly
(RunAtLoad fired, runs=9) but every spawn exited 1 because a previously
started daemon held the pidfile, and nothing recorded why.

Fatal errors on the daemon path now go through tracing::error!, reaching
both the log file and stderr. Other subcommands keep eprintln!, since
their stderr is the user's terminal.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EGntTHCW3sEPy1VBRopNNp

crates/xy/src/main.rs

@@ -92,7 +92,9 @@ async fn main() -> std::process::ExitCode {
     let filter = tracing_subscriber::EnvFilter::try_from_default_env()
         .unwrap_or_else(|_| tracing_subscriber::EnvFilter::new("info"));
 
-    if matches!(cli.cmd, Cmd::Daemon) {
+    let is_daemon = matches!(cli.cmd, Cmd::Daemon);
+
+    if is_daemon {
         use tracing_subscriber::fmt::writer::MakeWriterExt;
 
         if let Err(err) = paths.ensure_dirs() {
@@ -134,8 +136,15 @@ async fn main() -> std::process::ExitCode {
 
     match result {
         Ok(code) => std::process::ExitCode::from(code as u8),
-        Err(e) => {
-            eprintln!("xy: {e:#}");
+        Err(err) => {
+            // Under launchd the daemon's stderr is discarded, so a fatal
+            // startup error is only diagnosable if it reaches the log file.
+            if is_daemon {
+                tracing::error!("{err:#}");
+            } else {
+                eprintln!("xy: {err:#}");
+            }
+
             std::process::ExitCode::from(1)
         }
     }

crates/xy/tests/daemon_log.rs

@@ -0,0 +1,30 @@
+mod common;
+use common::*;
+
+#[tokio::test]
+async fn fatal_startup_error_is_written_to_daemon_log() {
+    let xy = xy_bin();
+    let mut h = Harness::new();
+
+    h.start_daemon(&xy).await;
+
+    let log = h.state_dir.join("logs/daemon.log");
+
+    let before = std::fs::read_to_string(&log).unwrap_or_default();
+
+    assert!(
+        !before.contains("another xy daemon"),
+        "log already reports contention before the second daemon ran: {before}"
+    );
+
+    let (code, _out, _err) = h.run_cli(&xy, &["daemon"]).await;
+
+    assert_eq!(code, 1, "second daemon should exit 1 on pidfile contention");
+
+    let after = std::fs::read_to_string(&log).expect("daemon.log should exist");
+
+    assert!(
+        after.contains("another xy daemon"),
+        "daemon.log must record why the daemon refused to start, got: {after:?}"
+    );
+}