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/user-dirs.dirs

@@ -0,0 +1,16 @@
+# This file is written by xdg-user-dirs-update
+# If you want to change or add directories, just edit the line you're
+# interested in. All local changes will be retained on the next run.
+# Format is XDG_xxx_DIR="$HOME/yyy", where yyy is a shell-escaped
+# homedir-relative path, or XDG_xxx_DIR="/yyy", where /yyy is an
+# absolute path. No other format is supported.
+# 
+XDG_DESKTOP_DIR="$HOME/.desktop"
+XDG_DOWNLOAD_DIR="$HOME/dwn"
+XDG_TEMPLATES_DIR="$HOME/.ignoreme"
+XDG_PUBLICSHARE_DIR="$HOME/.ignoreme"
+XDG_DOCUMENTS_DIR="$HOME/doc"
+XDG_MUSIC_DIR="$HOME/mus"
+XDG_PICTURES_DIR="$HOME/pic"
+XDG_VIDEOS_DIR="$HOME/vid"
+XDG_PROJECTS_DIR="$HOME/wrk"