refactor(nixos): declare bootloader per-host, not in workstation.nix

The bootloader is firmware-specific, not form-factor: UEFI hosts use
systemd-boot, BIOS hosts use GRUB. Drop boot.loader.systemd-boot.enable from
workstation.nix and declare it on the MBP instead, so the incoming BIOS-only
T400 (GRUB) doesn't have to force it off.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

system/machine/MBP-Asahi/configuration.nix

@@ -7,7 +7,9 @@
     ./hardware-configuration.nix
   ];
 
-  # Asahi manages the EFI vars from macOS; do not touch them from NixOS.
+  # UEFI boot via systemd-boot. Asahi manages the EFI vars from macOS, so do not
+  # touch them from NixOS.
+  boot.loader.systemd-boot.enable = true;
   boot.loader.efi.canTouchEfiVariables = false;
 
   networking.hostName = "Emma-Asahi";