fix(#2284): replace nixos-container kill with machinectl kill SIGKILL

nixos-container has no kill verb. The KillContainer priv request was
calling nixos-container kill which always fails. Replace with:

  machinectl kill <name> --signal=SIGKILL

which sends SIGKILL to all processes in the container — the correct
semantics for a forced shutdown (called after graceful stop has been
attempted). Add a machinectl_run helper alongside container_run so
callers stay consistent.

hive-sh4re/src/priv_proto.rs

@@ -230,7 +230,8 @@ pub enum PrivRequest {
     /// `nixos-container stop <name>`
     StopContainer { name: String },
 
-    /// `nixos-container kill <name>`
+    /// `machinectl kill <name> --signal=SIGKILL` — force-kills all processes
+    /// in the container. nixos-container has no kill verb.
     KillContainer { name: String },
 
     /// `nixos-container update <name> --flake <flake_ref>`