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/GOALS.md

@@ -22,7 +22,7 @@ confidence, default identity, release bar) live in
    ergonomic writer for the flake. Where a toggle can take effect without a
    rebuild, it must (`--no-switch` + flip the running service).
 4. **Beautiful.** One JSON themes the entire desktop coherently — Hyprland,
-   Waybar, Ghostty, btop, rofi, GTK/Qt, boot splash, greeter. Every new
+   Waybar, Kitty, btop, rofi, GTK/Qt, boot splash, greeter. Every new
    surface follows the palette. Informative, self-gating Waybar modules
    (they hide when irrelevant). No unthemed corner survives contact with
    the theme switcher.