nvim: make copilot host-conditional

Taking the work laptop's init.lua as canonical carried its commented-out
copilot line to every machine. Copilot was enabled on mainframe, system76
and the NAS before the refit — only the work laptop had it off. Sharing the
file silently disabled it on the other three.

The plugin list is now built in a table and copilot appended unless the
host is sctfw004. Keyed on vim.fn.hostname() rather than $DOTFILES_HOST,
because nvim launched from a desktop entry does not inherit the shell's
environment.

common/nvim/init.lua

@@ -49,21 +49,33 @@ vim.g.leader = " "
 -- PlugIns
 -- ------------------------------------------------------------------------------
 
-vim.pack.add({
+-- Host-conditional plugins. vim.fn.hostname() rather than an env var, because
+-- nvim launched from a desktop entry does not inherit the shell's environment.
+local host = vim.fn.hostname():lower()
+
+local plugins = {
     { src = "https://github.com/vague2k/vague.nvim" },
     { src = "https://github.com/mason-org/mason.nvim" },
     { src = "https://github.com/neovim/nvim-lspconfig" },
     { src = "https://github.com/stevearc/oil.nvim" },
     { src = "https://github.com/nvim-telescope/telescope.nvim" },
     { src = "https://github.com/nvim-lua/plenary.nvim" },
-    -- { src = "https://github.com/github/copilot.vim" },
     { src = "https://github.com/kdheepak/lazygit.nvim" },
     { src = "https://github.com/tpope/vim-surround" },
     { src = "https://github.com/tpope/vim-repeat" },
     { src = "https://github.com/tommcdo/vim-lion" },
     { src = "https://github.com/andythigpen/nvim-coverage" },
     { src = "https://github.com/nvim-treesitter/nvim-treesitter" },
-})
+}
+
+-- Copilot is off on the work laptop and on everywhere else. It was enabled on
+-- mainframe, system76 and the NAS before the refit; taking the work laptop's
+-- init.lua as canonical would otherwise have silently disabled it on all three.
+if host ~= "sctfw004" then
+    table.insert(plugins, { src = "https://github.com/github/copilot.vim" })
+end
+
+vim.pack.add(plugins)
 
 require("mason").setup()
 require("oil").setup()