pin internal/meta's hyperhive on the controller's rev and rebuild agents when it moves

The swarm controller offers its own running rev (services.hyperhive.c0re.
hyperhiveRev) as the hyperhive pin of internal/meta and takes it only when
the forge compare API shows it descends from the committed pin; merges to
hyperhive main no longer move the pin, and the first commit pins main's head.
When a commit moves the pin, every placed agent gets a TriggerDeploy.

A deploy after a meta update no longer waits on it succeeding (after_any), so
a down builder does not hold back hives that render locally.

nix/host-modules/swarm-controller.nix

@@ -286,6 +286,9 @@ let
       # helper answers only `https` for the forge.
       git_base = "https://${forgeCfg.domain}";
       github_disabled = !swarmCfg.github.enable;
+      # The rev this controller runs, which `internal/meta` pins hyperhive
+      # to once it descends from the committed pin; null on a dirty tree.
+      hyperhive_rev = config.services.hyperhive.c0re.hyperhiveRev;
     };
   };