feat(home): mkDefault behaviour settings so downstream can plain-override

Nomarchy's Hyprland/Ghostty/Waybar settings were set at normal priority,
so a downstream `home.nix` setting any value Nomarchy already set threw a
"conflicting definition values" error — overriding required lib.mkForce,
contradicting the README's "mkDefault throughout" claim.

Now: behaviour/chrome leaves (input, misc, monitor, animations, dwindle,
gesture, decoration.blur sizing, ghostty padding/decoration, waybar
settings+style) are lib.mkDefault — a plain home.nix assignment wins.
Appearance values that flow from theme-state.json (gaps, colors,
rounding, opacity, fonts) stay at normal priority on purpose: change them
via nomarchy-theme-sync (their source of truth) or lib.mkForce to
hardcode. bind/exec-once lists stay normal priority so additions
concatenate rather than replace.

Verified: behaviour/ghostty plain overrides build; theme values still
conflict (steering to the CLI); default rendered config is byte-identical.

New docs/OVERRIDES.md with the appearance→CLI / behaviour→home.nix /
component→toggle model and worked examples; linked from the README.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

modules/home/ghostty.nix

@@ -30,12 +30,14 @@ in
       palette = lib.imap0 (i: color: "${toString i}=${color}") t.ansi;
 
       # ── Chrome ────────────────────────────────────────────────────
+      # background-opacity is theme-driven (normal priority — use the
+      # CLI); the rest are mkDefault so a plain home.nix value wins.
       background-opacity = t.ui.terminalOpacity;
-      window-padding-x = 12;
-      window-padding-y = 12;
-      window-decoration = false;
-      gtk-single-instance = true;
-      confirm-close-surface = false;
+      window-padding-x = lib.mkDefault 12;
+      window-padding-y = lib.mkDefault 12;
+      window-decoration = lib.mkDefault false;
+      gtk-single-instance = lib.mkDefault true;
+      confirm-close-surface = lib.mkDefault false;
     };
   };
 }