docs: establish domain model, MVI spec, and environment reference

Capture the design work for the NixOS migration before any implementation:

- .claude/CONTEXT.md: domain glossary (Host, Module, Skeleton, Auto-loader,
  Enable convention, unstable/stable overlay)
- .claude/adr/0001-sops-nix-for-secrets.md: secrets tooling decision
- .claude/spec/laptop-mvi.md: frozen minimum-viable-install spec for neogaia

- reference/: read-only snapshot of the current CachyOS configs (secrets and
  state excluded), plus ENVIRONMENT.md profiling the live environment to guide
  replication

reference/home/.config/gtk-3.0/settings.ini

@@ -0,0 +1,18 @@
+[Settings]
+gtk-application-prefer-dark-theme=true
+gtk-button-images=true
+gtk-cursor-blink=true
+gtk-cursor-blink-time=1000
+gtk-cursor-theme-name=breeze_cursors
+gtk-cursor-theme-size=24
+gtk-decoration-layout=icon:minimize,maximize,close
+gtk-enable-animations=true
+gtk-font-name=Noto Sans,  14
+gtk-icon-theme-name=breeze-dark
+gtk-menu-images=true
+gtk-modules=colorreload-gtk-module:window-decorations-gtk-module
+gtk-primary-button-warps-slider=true
+gtk-sound-theme-name=ocean
+gtk-theme-name=Breeze
+gtk-toolbar-style=3
+gtk-xft-dpi=98304