feat(mibook): decrypt placeholder secret via host age key

Proves the sops-nix pipeline end to end on mibook: encrypts a
throwaway secret to the operator + mibook host keys, declares it in
machines/mibook/secrets.nix, and verifies it decrypts to
/run/secrets/placeholder (root:root, 0400) on nixos-rebuild switch.

Also removes the dead `let secrets = config.sops.secrets;` binding
from both environments.nix files — leftover from an earlier abandoned
plan, never referenced.

machines/mibook/environments.nix

@@ -1,9 +1,5 @@
 # enabled profiles
-{ config, lib, ... }:
-let
-  secrets = config.sops.secrets;
-in
-{
+_: {
   my.profiles = {
     kde-desktop.enable = true;
     zsh.enable = true;