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/fish/config.fish

@@ -0,0 +1,17 @@
+source /usr/share/cachyos-fish-config/cachyos-config.fish
+
+set -gx EDITOR nvim
+set -gx VISUAL nvim
+
+# overwrite greeting
+# potentially disabling fastfetch
+#function fish_greeting
+#    # smth smth
+#end
+
+# bun
+set --export BUN_INSTALL "$HOME/.bun"
+set --export PATH $BUN_INSTALL/bin $PATH
+
+# Pi
+fish_add_path "/home/alexion/.local/opt/node-v24.18.0-linux-x64/bin"