refactor: self-bootstrap lazy.nvim instead of chezmoi archive external

Replace the chezmoiexternal tarball install with the standard
lazy.nvim bootstrap snippet (clone-if-missing, pinned to v11.17.5,
the current latest stable tag). Gives lazy.nvim a real git checkout
instead of an extracted tarball, and drops chezmoi from the loop
entirely for this one piece - bumping the version is now just
editing the --branch tag in init.lua.

.chezmoiexternal.toml.tmpl

@@ -1,8 +0,0 @@
-# chezmoi:template:left-delimiter=#{{
-#{{- /* vim: set filetype=toml: */ -}}
-[".local/share/nvim/lazy.nvim"]
-    type = "archive"
-    url = "https://github.com/folke/lazy.nvim/archive/refs/tags/v11.17.5.tar.gz"
-    exact = true
-    stripComponents = 1
-    refreshPeriod = "2196h"

dot_config/nvim/init.lua

@@ -1,5 +1,24 @@
 require("core.config")
-vim.opt.runtimepath:prepend(vim.fn.stdpath("data") .. "/lazy.nvim")
+
+local lazypath = vim.fn.stdpath("data") .. "/lazy.nvim"
+if not vim.loop.fs_stat(lazypath) then
+    local out = vim.fn.system({
+        "git", "clone", "--filter=blob:none",
+        "--branch=v11.17.5",
+        "https://github.com/folke/lazy.nvim.git",
+        lazypath,
+    })
+    if vim.v.shell_error ~= 0 then
+        vim.api.nvim_echo({
+            { "Failed to clone lazy.nvim:\n", "ErrorMsg" },
+            { out, "WarningMsg" },
+        }, true, {})
+        vim.fn.getchar()
+        os.exit(1)
+    end
+end
+vim.opt.runtimepath:prepend(lazypath)
+
 require("lazy").setup({
     spec = {
         { import = "plugins" },