Add Ghostty, GNOME Tweaks, and a GNOME dark-mode default

Ghostty joins the cross-platform desktop app list (Linux-only in nixpkgs
today, filtered out on darwin by the existing availableOn check).
GNOME Tweaks is gated on osConfig.services.desktopManager.gnome.enable
rather than plain isLinux, since modules/home/desktop-apps.nix is also
consumed by the standalone, non-NixOS tom@linux home-manager profile,
which has no GNOME involved. Every NixOS host now also gets a dconf
default of org/gnome/desktop/interface color-scheme=prefer-dark, left
unlocked so GNOME Settings can still switch back to light mode.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HngxrvqEzq8oBEBDtrYJyB

1 files changed, 12 insertions, 0 deletions

diff --git a/openspec/changes/add-ghostty-gnome-tweaks-dark-mode/specs/gnome-desktop/spec.md b/openspec/changes/add-ghostty-gnome-tweaks-dark-mode/specs/gnome-desktop/spec.md
new file mode 100644
index 0000000..f9344b4
--- /dev/null
+++ b/openspec/changes/add-ghostty-gnome-tweaks-dark-mode/specs/gnome-desktop/spec.md
@@ -0,0 +1,12 @@
+## ADDED Requirements
+
+### Requirement: GNOME defaults to dark mode
+Every NixOS host built from this flake's shared system module SHALL configure its GNOME desktop session to use dark mode (`color-scheme: prefer-dark`) as the default appearance, while leaving the setting changeable by the user.
+
+#### Scenario: Fresh session uses dark mode
+- **WHEN** a user who has not previously changed the GNOME appearance setting logs into a NixOS host built from this flake's shared system module
+- **THEN** the GNOME desktop session appears in dark mode
+
+#### Scenario: User can still switch to light mode
+- **WHEN** a user opens GNOME Settings' Appearance page on a host built from this flake's shared system module
+- **THEN** the user can select light mode, and that choice takes effect and persists across future logins