feat: migrate from lazy.nvim to vim.pack

Replace lazy.nvim plugin manager with Neovim 0.12's native vim.pack API.
All plugin config files rewritten from lazy.nvim spec tables to imperative
require/setup format with explicit vim.keymap.set() calls.

Key changes:
- vim.pack.add() with ~53 plugins in init.lua
- blink.cmp/pairs/download pinned to version tags (vim.version.range)
- PackChanged autocmd for markdown-preview build hook
- Ordered requires: colorscheme → ui → treesitter → completion → lsp → rest
- Plugin setup guards (gitsigns, which-key, blink.cmp) handle deferred
  plugin/ file loading correctly

Net reduction: ~438 lines across 13 files.

1 files changed, 19 insertions, 40 deletions

diff --git a/home/.config/nvim/lua/plugins/ai.lua b/home/.config/nvim/lua/plugins/ai.lua
index 5936339..d88bc7e 100644
--- a/home/.config/nvim/lua/plugins/ai.lua
+++ b/home/.config/nvim/lua/plugins/ai.lua
@@ -1,44 +1,23 @@
-return {
-  {
-    "zbirenbaum/copilot.lua",
-    cmd = "Copilot",
-    build = ":Copilot auth",
-    event = "InsertEnter",
-    dependencies = {
-      {
-        "copilotlsp-nvim/copilot-lsp",
-        init = function()
-          vim.g.copilot_nes_debounce = 500
-        end,
+require("copilot").setup({
+  suggestion = { enabled = false },
+  panel = { enabled = false },
+  server_opts_overrides = {
+    settings = {
+      telemetry = {
+        telemetryLevel = "off",
       },
     },
-    keys = {
-      {
-        "<leader>tc",
-        function()
-          require("copilot.command").toggle()
-        end,
-        desc = "[T]oggle [C]opilot attachment",
-      },
-    },
-    opts = {
-      suggestion = { enabled = false },
-      panel = { enabled = false },
-      server_opts_overrides = {
-        settings = {
-          telemetry = {
-            telemetryLevel = "off",
-          },
-        },
-      },
-      nes = {
-        enabled = true,
-        keymap = {
-          accept_and_goto = "<leader>p",
-          accept = false,
-          dismiss = "<Esc>",
-        },
-      },
+  },
+  nes = {
+    enabled = true,
+    keymap = {
+      accept_and_goto = "<leader>p",
+      accept = false,
+      dismiss = "<Esc>",
     },
   },
-}
+})
+
+vim.keymap.set("n", "<leader>tc", function()
+  require("copilot.command").toggle()
+end, { desc = "[T]oggle [C]opilot attachment" })