gpu-worker: feeder-paused state in (health) RPC

bend's (health) now reports whether $BEND_QUEUE_DIR/FEEDER_PAUSE
marker is set. Consumers (feeder, factory-status, ops scripts) get
pause state in the same single RPC as bend liveness + supervisor
proc counts + queue depths.

Returns:
  1   marker present (operator wants this host out of rotation)
  0   no marker (host in active rotation)
  -1  BEND_QUEUE_DIR env unset (host has no associated queue)

Lets a future feeder version drop separate SSH pause-marker probes
in favor of the bend health RPC. Today's feeder still does the SSH
check; this just opens the door for the simpler model.

examples/cuda-fanout/gpu-worker.lsp

@@ -449,6 +449,16 @@
                 ((or (eq? line #f) (eof-object? line)) 0)
                 (else (or (string->number (parse-trim line)) 0)))))))))))
 
+(define (health-feeder-paused)
+  ;; 1 if $BEND_QUEUE_DIR/FEEDER_PAUSE marker present (operator wants
+  ;; this host out of the rotation; feeder should not push or restart
+  ;; supervisors here). 0 if no marker. -1 if BEND_QUEUE_DIR unset.
+  (let ((qdir (get-environment-variable "BEND_QUEUE_DIR")))
+    (cond
+      ((or (eq? qdir #f) (string=? qdir "")) -1)
+      ((file-exists? (string-append qdir "/FEEDER_PAUSE")) 1)
+      (else 0))))
+
 (define (handle-health)
   (list 'ok
         (list 'load-avg (health-load-avg))
@@ -458,7 +468,8 @@
         (list 'dispatcher-procs (health-dispatcher-procs))
         (list 'queue-ready (health-queue-count "ready"))
         (list 'queue-emitting (health-queue-count "emitting"))
-        (list 'queue-done (health-queue-count "done"))))
+        (list 'queue-done (health-queue-count "done"))
+        (list 'feeder-paused (health-feeder-paused))))
 
 ;;; -- dispatch --------------------------------------------------