request the review classifier's review when a PR's head moves

The router gets a request_reviewer node and a pull_request_sync source;
the built-in head-classifier flow asks swarm-controller, over
$SWARM.router.sink.request-reviewer, to request review-classifier on
every push, unless its review of the head already stands. The controller
subscribes to pull_request_sync on its pull-request hook and answers
the router; nats-auth grants the router the publish and the controller
the subscribe.

frontend/packages/swarm-ui/test/flow-types.test.mjs

@@ -73,6 +73,14 @@ test("describeNode: says what each kind does", () => {
     "clear todos forge:{repo}#{number}:* of {todo_holders}",
   );
   assert.equal(describeNode(flow.nodes[4]), "todo to triage, key k: look");
+  assert.equal(
+    describeNode({
+      id: "r",
+      kind: "request_reviewer",
+      reviewer: "review-classifier",
+    }),
+    "request a review from review-classifier",
+  );
 });
 
 test("withBuiltin: switching off adds once, switching on removes", () => {