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/navigation.lua

@@ -0,0 +1,27 @@
+return {
+  {
+    "stevearc/oil.nvim",
+    lazy = false,
+    opts = {
+      view_options = { show_hidden = true },
+    },
+    keys = {
+      { "<leader>e", "<cmd>Oil<CR>", desc = "Open file browser" },
+    },
+  },
+  {
+    "folke/snacks.nvim",
+    priority = 1000,
+    lazy = false,
+    opts = {
+      picker = { enabled = true },
+      notifier = { enabled = true },
+      input = { enabled = true },
+    },
+    keys = {
+      { "<leader>f", function() require("snacks").picker.files() end, desc = "Find files" },
+      { "<leader>s", function() require("snacks").picker.grep() end, desc = "Search text" },
+      { "<leader>b", function() require("snacks").picker.buffers() end, desc = "Switch buffer" },
+    },
+  },
+}