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/KEYBINDINGS.md

@@ -9,6 +9,7 @@ rebuild, never the generated dotfiles.
 | Sway (compositor) | [`sway.nix`](./sway.nix) `config.keybindings` + `config.modes`, plus the home-manager Sway module's built-in defaults |
 | tmux              | [`shell.nix`](./shell.nix) `programs.tmux`                                                                            |
 | zsh line editor   | [`shell.nix`](./shell.nix) `programs.zsh.historySubstringSearch`                                                      |
+| Neovim            | [`editor.nix`](./editor.nix) `programs.nixvim`                                                                        |
 | foot (terminal)   | foot package defaults — only colours are themed (in `sway.nix`)                                                       |
 
 **Conventions**
@@ -167,6 +168,26 @@ Only colours are themed; these are foot's default key bindings.
 
 ---
 
+## Neovim
+
+Leader is **`Space`**. `Ctrl`+`h/j/k/l` is shared with tmux (see above): it moves
+across vim splits and tmux panes seamlessly. Everything else is stock vim, plus:
+
+| Shortcut               | Action                                                    |
+| ---------------------- | --------------------------------------------------------- |
+| `,``,`                 | Toggle the file tree (nvim-tree) — comma pressed twice    |
+| `Ctrl`+`h`/`j`/`k`/`l` | Move between vim splits / tmux panes (vim-tmux-navigator) |
+| `gd`                   | Go to definition (LSP)                                    |
+| `gr`                   | List references (LSP)                                     |
+| `K`                    | Hover documentation (LSP)                                 |
+| `<leader>rn`           | Rename symbol (LSP; `<leader>` is `Space`)                |
+| `<leader>ca`           | Code action (LSP)                                         |
+
+LSP covers Nix, Lua, Python and Terraform (the work box adds C# and Helm);
+completion (nvim-cmp) appears as you type. `:Git` opens fugitive.
+
+---
+
 ## zsh
 
 | Shortcut  | Action                                                                                             |