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

CLAUDE.md

@@ -174,8 +174,8 @@ Swarm-level:
   `nix/packages/swarm-ui.nix`, not in `packages.default`'s closure).
 - **`nix/`** — `host-modules/` (the host stack), `agent-modules/`
   (per-agent harness features), `container-modules/` (guest side of
-  service containers), `templates/{agent,ruth}.nix` (container entry
-  points), `packages/`, `docs/` (options doc), `module-eval/` (NixOS
+  service containers), `templates/agent.nix` (the container entry
+  point), `packages/`, `docs/` (options doc), `module-eval/` (NixOS
   module-eval tests), `builder/` (`check-flake`), `script-tests/`, plus
   `sources.nix` / `rust.nix` / `checks.nix` / `devshell.nix` /
   `treefmt.nix` behind the thin `flake.nix`.