Strip magic commands before forwarding "execute_request" messages.

* guix-jupyter-kernel.scm (execute-request-sans-magic): New procedure.
(reply-execute-request): In ";;guix environment NAME" requests, call it.

guix-jupyter-kernel.scm

@@ -212,6 +212,20 @@ to KERNEL as a reply to MESSAGE, and return STATE suitably adjusted."
        ;; TODO: Send "error".
        state))))
 
+
+(define (execute-request-sans-magic message)
+  "Return MESSAGE, an 'execute_request' message, with its \";;guix\" magic
+stripped."
+  (let* ((content  (json-string->scm (message-content message)))
+         (code     (assoc-ref content "code"))
+         (stripped (string-drop code
+                                (or (string-index code #\newline)
+                                    (string-length code)))))
+    (set-message-content message
+                         (scm->json-string
+                          `(("code" . ,stripped)
+                            ,@(alist-delete "code" content))))))
+
 (define (reply-execute-request kernel kind message state)
   (let* ((content (message-content message))
          (code    (assoc-ref (json-string->scm content) "code"))
@@ -248,7 +262,7 @@ to KERNEL as a reply to MESSAGE, and return STATE suitably adjusted."
          ((? kernel? proxy)
           (format/log "evaluating code in container ~s (PID ~s)~%"
                       name (kernel-pid proxy))
-          (send-message proxy message)
+          (send-message proxy (execute-request-sans-magic message))
           (let ((state (increment-execution-count state)))
             (set-proxy-state-default-environment state name)))))
       ((";;guix" _ ...)