use full event when signing with the extension.

because if the client is building the event with the pubkey A, but
the actual key used by the extension is B, using just the `.sig`
property of the value returned by `window.nostr.signEvent()` will
cause the event to be invalid.

src/agent/keys.ts

@@ -28,11 +28,13 @@ const sign = async event => {
 
   event.pubkey = get(pubkey)
   event.id = getEventHash(event)
-  event.sig = ext
-    ? prop('sig', await ext.signEvent(event))
-    : signEvent(event, get(privkey))
 
-  return event
+  if (ext) {
+    return await ext.signEvent(event)
+  } else {
+    event.sign = signEvent(event, get(privkey))
+    return event
+  }
 }
 
 const getCrypt = () => {