feat(nvim): migrate editor from vim to Neovim (parity + LSP)

Rewrite editor.nix on programs.nixvim, keeping every prior feature: file
tree (nvim-tree, toggle ,,), indent guides (indent-blankline), fugitive,
vim-tmux-navigator, Catppuccin Mocha, 2-space hard tabs, and the
*Jenkinsfile=groovy rule. Replace the inert ALE with a real LSP stack —
nvim-lspconfig, nvim-cmp completion, and tree-sitter for highlighting.
Leader is Space; LSP keymaps gd/gr/K/<leader>rn/<leader>ca.

Universal servers: nil (Nix), lua_ls, pyright (Python), terraformls. The
work box (work.nix) additionally enables omnisharp (C#) and helm_ls (Helm),
so the heavy omnisharp closure stays off the personal machines.

default.nix drops VISUAL=vim so nixvim's defaultEditor owns $EDITOR/$VISUAL.
README and KEYBINDINGS updated (and two stale references corrected).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lyrathorpe/home/work.nix

@@ -52,4 +52,13 @@
   programs.go = {
     enable = true;
   };
+
+  # LSP servers only relevant to work: C# (omnisharp) and Helm charts (helm_ls).
+  # The shared editor (lyrathorpe/home/editor.nix) carries the universal ones;
+  # these are gated to this host so the heavy omnisharp closure stays off the
+  # personal machines. Tree-sitter grammars (highlighting) remain global there.
+  programs.nixvim.plugins.lsp.servers = {
+    omnisharp.enable = true;
+    helm_ls.enable = true;
+  };
 }