fix(worker): report startup port conflicts instead of crashing silently

Mutex was Local\ (per Windows session), so a second RDP/fast-user-switching
session bypassed the single-instance guard. A Kestrel bind failure on top of
that had no try/catch, so a second worker (or any process on 47821/47822)
died as an unhandled crash with nothing in the log to explain why the UI
never connects. Mutex is now Global\, bind failures and any last-resort
unhandled exception log a Fatal line and exit(1) instead of crashing raw.

src/ClaudeDo.Worker/Lifecycle/WorkerStartupFailure.cs

@@ -0,0 +1,19 @@
+namespace ClaudeDo.Worker.Lifecycle;
+
+/// <summary>
+/// Formats the Fatal log line for a startup/bind failure (e.g. a port already in use) so
+/// Program.cs can log a diagnosable message and exit cleanly instead of crashing unhandled.
+/// </summary>
+public static class WorkerStartupFailure
+{
+    public const int ExitCode = 1;
+
+    public static string FormatBindFailureMessage(Exception ex, int signalRPort, int externalMcpPort)
+    {
+        var ports = externalMcpPort > 0
+            ? $"ports {signalRPort} and {externalMcpPort}"
+            : $"port {signalRPort}";
+        return $"ClaudeDo.Worker failed to start on {ports}: {ex.Message} " +
+               "Is another worker instance already running, or is another process using this port?";
+    }
+}