Establish project foundation: domain model, MVI spec, 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/xsettingsd/xsettingsd.conf

@@ -0,0 +1,17 @@
+Net/CursorBlinkTime 1000
+Net/CursorBlink 1
+Gdk/UnscaledDPI 98304
+Gdk/WindowScalingFactor 1
+Gtk/EnableAnimations 1
+Gtk/DecorationLayout "icon:minimize,maximize,close"
+Net/ThemeName "Breeze-Dark"
+Gtk/PrimaryButtonWarpsSlider 1
+Gtk/ToolbarStyle 3
+Gtk/MenuImages 1
+Gtk/ButtonImages 1
+Gtk/CursorThemeSize 24
+Gtk/CursorThemeName "breeze_cursors"
+Net/SoundThemeName "ocean"
+Net/IconThemeName "breeze-dark"
+Gtk/FontName "Noto Sans,  14"
+