hive-priv: remove the RestartMatrixDaemon command

Its only client was hive-c0re's matrix-account-login handler, removed
earlier on this branch, so nothing sends `restart_matrix_daemon` any
more. Drops the `PrivRequest` variant, the hive-priv handler and its
`systemctl --machine=h-<agent> restart hive-matrix-daemon.service`
helper, and the row in the hive-priv op table in security.md.

`PrivRequest` is internally tagged by `op` name, so no other variant's
encoding changes. A new token file still re-fires the daemon through
its `matrix-token*` path unit.

Refs #4348

hive-priv-sock/src/lib.rs

@@ -562,15 +562,6 @@ pub enum PrivRequest {
         label: String,
     },
 
-    /// Restart `hive-matrix-daemon.service` inside an agent container via
-    /// `systemctl --machine=h-<agent_name> restart hive-matrix-daemon.service`.
-    /// Used by hive-c0re to kick the daemon after a successful token write
-    /// so it picks up the new credential without a full container restart.
-    RestartMatrixDaemon {
-        /// Logical agent name (validated by `validate_agent_name`).
-        agent_name: String,
-    },
-
     /// Register the hive-ci Forgejo Actions runner: write the registration
     /// token to the host-side `/run/hive-ci/runner-token` env-file (root-owned,
     /// bind-mounted read-only into the container) as `TOKEN=<token>`, then