full-setup vm test: spawn, a reviewer-archetype agent, replaces ruth

mara chose on the PR to test a reviewer-archetype agent named spawn first
and leave a custom root agent (ruth) for later. So the test creates spawn
with the archetype's default permissions and drops manage_root_agent, which
only ruth carried.

- hive-meta-flake/testdata/meta-flake-swarm.nix.in: the agent is spawn, and
  its web port is 8316, the port the name hashes to (ruth's was 8232).
- hive-meta-flake/src/lib.rs: the unit test that compares this fixture with
  the rendered flake renders spawn instead of ruth.
- nix/vm-tests/full-setup.nix: the agents list and step 8 use spawn.
- The `swarmctl setup status` expectation keeps its "9  ruth created and up"
  row: that is the product's fixed step text (swarmctl/src/setup.rs), not the
  test's agent. The test creates no root agent, so step 9 is expected to be
  not done, and the subtest's comment says so.

hive-meta-flake/src/lib.rs

@@ -604,7 +604,7 @@ mod tests {
             "@HYPERHIVE_URL@",
             &[
                 spec("iris", archetype(), None),
-                spec("ruth", archetype(), None),
+                spec("spawn", archetype(), None),
             ],
             &vars,
             false,

hive-meta-flake/testdata/meta-flake-swarm.nix.in

@@ -87,7 +87,7 @@
     {
       nixosConfigurations = {
         iris = mkAgent { name = "iris"; port = 8178; archetype = { source = "hyperhive"; name = "reviewer"; }; };
-        ruth = mkAgent { name = "ruth"; port = 8232; archetype = { source = "hyperhive"; name = "reviewer"; }; };
+        spawn = mkAgent { name = "spawn"; port = 8316; archetype = { source = "hyperhive"; name = "reviewer"; }; };
       };
     };
 }