cli: emit fiwi: start only when stderr is a TTY.

Avoids interleaving the banner with captured stdout (e.g. SSH
port-metrics-json and check_concentrator table output).

Made-with: Cursor

fiwi/cli.py

@@ -206,7 +206,9 @@ def main() -> int:
     if rc_ssh_map is not None:
         return rc_ssh_map
 
-    os.write(2, b"fiwi: start\n")
+    # Skip when stderr is a pipe (SSH capture, subprocess): avoids interleaving with stdout consumers.
+    if sys.stderr.isatty():
+        os.write(2, b"fiwi: start\n")
     try:
         load_brainstem()
     except Exception as exc: