# mibook: choose terminal-only vs desktop at boot

## Goal

Let mibook boot to a text console by default, with KDE fully installed and
launchable on demand. The user decides after boot whether they need the
graphical desktop, without rebuilding the system.

## Behavior

- mibook boots to a text console (`multi-user.target`). SDDM and Plasma do
  **not** start automatically.
- KDE (SDDM + Plasma 6) remains fully installed.
- Running `desktop` in the terminal starts the SDDM display manager, which
  presents the graphical login screen; logging in there enters a Plasma 6
  session.
- No reverse command. To return to a text console, stop the display manager
  (`sudo systemctl stop display-manager`) or reboot.
- Other machines that enable the `kde-desktop` profile are unaffected; their
  desktop still starts on boot.

## Implementation

### `modules/environments/kde-desktop/default.nix`

Add an opt-out option to the existing profile:

- New option `my.profiles.kde-desktop.startOnBoot`, a boolean defaulting to
  `true`. The default preserves the current behavior for every consumer.
- The existing `config` block (SDDM, Plasma 6, user packages) stays enabled
  whenever the profile is enabled, regardless of `startOnBoot`.
- When `startOnBoot = false`, additionally apply:
  - `systemd.services.display-manager.wantedBy = lib.mkForce [ ];` so SDDM is
    defined but not pulled in by any boot target.
  - `systemd.defaultUnit = "multi-user.target";` so the boot target is the
    text console.
  - A `desktop` command provided via
    `pkgs.writeShellScriptBin "desktop" "exec sudo systemctl start display-manager.service"`,
    added to `environment.systemPackages`.

### `machines/mibook/environments.nix`

Set `kde-desktop.startOnBoot = false;` alongside the existing
`kde-desktop.enable = true;`.

## Testing / verification

- `nix build '.#nixosConfigurations.mibook.config.system.build.toplevel'`
  builds without error.
- After `nixos-rebuild switch` on mibook: system boots to a TTY, no SDDM;
  running `desktop` brings up the SDDM login and a working Plasma session.
- Confirm jupiter (or any non-mibook consumer) is unchanged: its evaluated
  `systemd.defaultUnit` / display-manager wantedBy are not altered.

## Trade-offs

- `desktop` relies on `sudo`; the user has sudo access, so no extra
  configuration is required.
- Chose "boot to terminal, launch on demand" over a GRUB boot-menu entry
  because it is more robust across NixOS generations and needs no fragile
  static kernel entries.