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/nvim/lua/vim_options.lua

@@ -0,0 +1,29 @@
+vim.g.mapleader = " "
+
+local opt = vim.opt
+
+-- Clipboard: use neovim's built-in OSC 52 provider, no external binary needed.
+vim.g.clipboard = "osc52"
+opt.clipboard = "unnamedplus"
+
+opt.number = true
+opt.relativenumber = true
+
+opt.shiftwidth = 2
+opt.tabstop = 2
+opt.expandtab = true
+
+opt.mouse = "a"
+
+opt.undofile = true
+
+opt.ignorecase = true
+opt.smartcase = true
+
+opt.splitright = true
+opt.splitbelow = true
+
+opt.wrap = false
+
+opt.scrolloff = 8
+opt.cursorline = true