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/plugins/ui.lua

@@ -0,0 +1,55 @@
+return {
+  {
+    "gbprod/nord.nvim",
+    lazy = false,
+    priority = 1000,
+    opts = {
+      transparent = true,
+    },
+    config = function(_, opts)
+      require("nord").setup(opts)
+      vim.cmd.colorscheme("nord")
+    end,
+  },
+  {
+    "MeanderingProgrammer/render-markdown.nvim",
+    ft = { "markdown" },
+    dependencies = { "nvim-treesitter/nvim-treesitter" },
+    opts = {},
+  },
+  {
+    "folke/which-key.nvim",
+    lazy = false,
+    config = true,
+  },
+  {
+    "nvim-treesitter/nvim-treesitter",
+    branch = "master",
+    build = ":TSUpdate",
+    opts = {
+      ensure_installed = {
+        "markdown",
+        "markdown_inline",
+        "lua",
+        "bash",
+        "fish",
+        "rust",
+        "javascript",
+        "typescript",
+        "java",
+        "kotlin",
+        "c",
+        "cpp",
+        "html",
+        "css",
+        "python",
+      },
+      auto_install = false,
+      highlight = { enable = true },
+      indent = { enable = true },
+    },
+    config = function(_, opts)
+      require("nvim-treesitter.configs").setup(opts)
+    end,
+  },
+}