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/.gitignore

@@ -0,0 +1,8 @@
+.dotfiles
+.DS_Store
+*.swp
+*.swo
+*~
+Thumbs.db
+**/__pycache__
+.config/fish/conf.d/secrets.fish