fix: nvim first boot on a fresh box — tree-sitter CLI, no luarocks, guarded local plugin

nvim-treesitter (main) hard-requires the tree-sitter CLI: new manifest
row via npm. hererocks failed compiling lua 5.1 (readline headers) for
oxocarbon's incidental rockspec: rocks disabled, the colorscheme works
without. The ~/code/nvim-ai-assist local plugin gets a cond so boxes
without the checkout skip it silently. All three found live, phase 5 VM.

private_dot_config/nvim/lua/plugins/ai_assist.lua

@@ -1,6 +1,9 @@
 return {
   dir = vim.fn.expand("~/code/nvim-ai-assist"),
   name = "nvim_ai_assist",
+  -- A local checkout that only some machines have. Without the cond, every
+  -- fresh box reports "Local plugin does not exist" on first sync.
+  cond = vim.fn.isdirectory(vim.fn.expand("~/code/nvim-ai-assist")) == 1,
   lazy = false,
   cmd = { "AI", "AIHealth", "AIRefreshContext" },
   keys = {