drop the manager socket, the no-op migrate branch and other ruth special cases

start_manager bound the same path register_agent binds. The migrate branch, hive-priv-sock::MANAGER_NAME and MANAGER_CONTAINER were no-ops or dead. The send allow-list docs promised a manager exemption the code never had. Tests that used ruth as the manager fixture use plain names.

hive-agent-mcp/src/send_allow.rs

@@ -7,8 +7,7 @@
 /// Where the NixOS module writes the per-agent send allow-list (see
 /// `nix/agent-modules/mcp.nix`). Empty list = unrestricted (the
 /// default). Non-empty list constrains `mcp__hyperhive__send`'s `to`
-/// field; the manager is always implicitly permitted regardless of
-/// the list contents.
+/// field.
 const SEND_ALLOW_PATH: &str = "/etc/hyperhive/send-allow.json";
 
 /// Enforce the per-agent send allow-list. Returns `Ok` when the