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/plugin.lua

@@ -0,0 +1,23 @@
+local lazypath = vim.fn.stdpath("data") .. "/lazy/lazy.nvim"
+if not vim.uv.fs_stat(lazypath) then
+  local lazyrepo = "https://github.com/folke/lazy.nvim.git"
+  local out = vim.fn.system({ "git", "clone", "--filter=blob:none", "--branch=stable", lazyrepo, lazypath })
+  if vim.v.shell_error ~= 0 then
+    vim.api.nvim_echo({
+      { "Failed to clone lazy.nvim:\n", "ErrorMsg" },
+      { out, "WarningMsg" },
+      { "\nPress any key to exit..." },
+    }, true, {})
+    vim.fn.getchar()
+    os.exit(1)
+  end
+end
+vim.opt.rtp:prepend(lazypath)
+
+require("lazy").setup({
+  spec = {
+    { import = "plugins" },
+  },
+  install = { colorscheme = { "nord" } },
+  checker = { enabled = false },
+})