fix(home): remove dead behavior options, reserve overrides API

Two declared-but-non-functional option subsystems in core/home were
documented in OPTIONS.md and actively misleading users.

1. `nomarchy.behavior.hyprland.{bindings,input,windowRules,autostart}`
   were declared in core/home/behavior.nix with a `behaviorConfigs`
   mapping let-binding — both completely unread elsewhere in the tree.
   The actual hypr/*.conf files are deployed by
   features/desktop/hyprland/default.nix with `lib.mkDefault`,
   unconditionally. Setting `behavior.hyprland.bindings = false` had
   zero effect. OPTIONS.md's "Disable Nomarchy's default Hyprland
   keybindings" example was a lie. Removed the four dead options,
   deleted behavior.nix entirely, dropped the import from
   core/home/default.nix, and rewrote the OPTIONS.md example to use
   `xdg.configFile."hypr/bindings.conf".source = ./mine` (which
   actually works against the existing `lib.mkDefault` priority).

2. `nomarchy.overrides.{enable,paths}` advertised a file-based override
   loader that doesn't exist. The module created
   `~/.config/nomarchy/overrides/{hypr,waybar,apps}` directories and
   wrote a README claiming "place files here to override upstream
   defaults" — but `getOverrideOrDefault` was never called and `paths`
   was never populated. Rewrote core/home/overrides.nix to keep just
   the option declarations (so configs that already set these still
   evaluate) and marked them clearly as reserved/no-op in OPTIONS.md.
   Removed the misleading README write and dir-creation. Logged a
   Next-column roadmap row for implementing the loader properly.

While here:
- Clarified `nomarchy.configOverrides` (the *working* bulk-redirect
  mechanism) vs `nomarchy.overrides.*` (the reserved one) in OPTIONS.md
  — they're different things and the "See Overrides below" link was
  pointing at the broken subsystem.
- Fixed OPTIONS.md `nomarchy.iconsTheme` / `nomarchy.isLightMode`
  default text — both are derived from the active theme in
  core/home/state.nix, not the static literals the docs claimed.
- Updated docs/AGENT.md §2 and docs/STRUCTURE.md to reflect the
  behavior.nix removal and the overrides.nix reservation.

Found during Pillar 8 audit of core/home modules.

core/home/default.nix

@@ -5,7 +5,6 @@
     ./options.nix
     ./state.nix
     ./overrides.nix
-    ./behavior.nix
     ./fonts.nix
     ./configs.nix
     ./security.nix