feat(terminal): Kitty only — drop Ghostty (#95)

Ghostty's OpenGL 4.3 floor broke the Acer (HD 4000 / 4.2). Dual-terminal
support (default Ghostty + install-time Kitty fallback) was more complexity
than it was worth.

- Remove ghostty.nix / nomarchy.ghostty.enable
- Kitty always installed and themed from theme-state
- Default nomarchy.terminal = kitty; SUPER+Return, doctor, calendar,
  what-changed use kitty --class=com.nomarchy.*
- Drop install-time glxinfo probe and mesa-demos on the installer
- Docs/REQUIREMENTS no longer list OpenGL 4.3 as a terminal floor

Verified: V0 flake check; option-docs; installer-safety; template HM has
kitty, no ghostty; menu/calendar emit classed kitty launches.

agent/THEME-DESIGN.md

@@ -20,7 +20,7 @@ consumes the palette from the state JSON, never a side file.
 ## Design responsibilities
 
 1. **Holistic design:** a theme spans Hyprland (borders, shadows,
-   animations), Waybar, Ghostty, btop, fastfetch, rofi, and wallpaper —
+   animations), Waybar, Kitty, btop, fastfetch, rofi, and wallpaper —
    one coherent identity, no unthemed corner.
 2. **Color theory:** create or adapt advanced palettes. Draw inspiration
    from established aesthetics (Everforest, Nord, Gruvbox) or r/unixporn