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/git.lua

@@ -0,0 +1,36 @@
+return {
+  {
+    "NeogitOrg/neogit",
+    dependencies = {
+      "nvim-lua/plenary.nvim",
+      "sindrets/diffview.nvim",
+    },
+    keys = {
+      {
+        "<leader>g",
+        function()
+          require("gitsigns").toggle_current_line_blame(true)
+          require("neogit").open()
+        end,
+        desc = "Open git (Neogit)",
+      },
+    },
+    config = function()
+      require("neogit").setup()
+      vim.api.nvim_create_autocmd("BufUnload", {
+        callback = function(args)
+          if vim.bo[args.buf].filetype == "NeogitStatus" then
+            require("gitsigns").toggle_current_line_blame(false)
+          end
+        end,
+      })
+    end,
+  },
+  {
+    "lewis6991/gitsigns.nvim",
+    event = "BufWinEnter",
+    opts = {
+      current_line_blame = false,
+    },
+  },
+}