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.

modules/home/theme.nix

@@ -6,7 +6,7 @@
 #
 # Theme changes are fully Home Manager managed: `nomarchy-theme-sync
 # apply <theme>` writes the JSON and runs `home-manager switch`, baking
-# everything (Hyprland, Waybar, Ghostty, btop, Stylix) into one
+# everything (Hyprland, Waybar, Kitty, btop, Stylix) into one
 # read-only generation. No runtime patching, no partial states; theme
 # history is generation history.
 #