build every agent from the agent-base template

Delete nix/templates/ruth.nix, the ruth nixosConfiguration, module and
toplevel, the hive-c0re managerToplevel option, and the meta flake's
isManager switch with lifecycle::MANAGER_NAME. Every agent, ruth
included, now builds from nix/templates/agent.nix. The template's only
content beyond agent.nix was docs.enable = mkDefault true; docs.enable
is off by default for every agent and ruth's own agent.nix turns it on.

Refs #5377

nix/host-modules/hive-c0re/default.nix

@@ -189,7 +189,7 @@ in
     # above for the rationale.
     environment.etc."hyperhive/serve.json".text = serveConfigJson;
 
-    # Pull the per-container toplevels into the host system closure.
+    # Pull the agent-base toplevel into the host system closure.
     # `system.extraDependencies` adds paths to the system build
     # without referencing them at runtime — nixos-rebuild fetches /
     # builds them, they end up in /nix/store, and the first
@@ -197,7 +197,6 @@ in
     # do. Gated because the closure is sizeable and pinned to x86_64.
     system.extraDependencies = lib.optionals cfg.preBuildAgentTemplates [
       cfg.agentBaseToplevel
-      cfg.managerToplevel
     ];
 
     # Unprivileged coordinator user. hive-c0re runs as this user;