[taskboard] isolate direct ssh from host config

lib/dispatch.ts

@@ -233,12 +233,18 @@ async function dispatchDirectTask(taskId: number): Promise<DispatchResult> {
   }
 
   const sshArgs = [
+    "-F",
+    "/dev/null",
     "-o",
     "BatchMode=yes",
     "-o",
     "StrictHostKeyChecking=accept-new",
     "-o",
     "ConnectTimeout=15",
+    "-o",
+    "IdentitiesOnly=yes",
+    "-o",
+    "UserKnownHostsFile=/tmp/taskboard_known_hosts",
     "-i",
     DIRECT_SSH_KEY_PATH,
     "-p",