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/fish/conf.d/env.fish

@@ -0,0 +1,4 @@
+set -gx EDITOR nvim
+set -x ANDROID_HOME $HOME/Android/Sdk
+fish_add_path $ANDROID_HOME/platform-tools
+fish_add_path $ANDROID_HOME/tools/bin