feat(editor): add nvim-cmp completion keymaps (#40)

Closes #39.

nvim-cmp ships no default keymaps, so the completion menu (including the path source) appeared but nothing could navigate or accept it.

Bind the usual set in `plugins.cmp.settings.mapping`:

- `<C-n>` / `<C-p>` and `<Tab>` / `<S-Tab>` — select next/previous
- `<C-Space>` — open the menu
- `<C-e>` — abort
- `<CR>` — confirm with `select = false` (bare Enter stays a newline unless an entry is highlighted)

Documentation: `KEYBINDINGS.md` gains a completion-menu table under the Neovim section covering these keys, and the Neovim summary is reworded accordingly.

Verified by rendering the generated nvim config: the mappings emit as raw Lua (e.g. `["<CR>"] = cmp.mapping.confirm({ select = false })`), not quoted strings.

---------

Co-authored-by: Emma Thorpe <emma.thorpe@citrix.com>
Reviewed-on: #40

lyrathorpe/home/KEYBINDINGS.md

@@ -188,10 +188,22 @@ across vim splits and tmux panes seamlessly. Everything else is stock vim, plus:
 | `<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. Files are formatted on save
-(conform-nvim). `:Git` opens fugitive; gitsigns shows gutter signs. which-key
-pops up after `<leader>` to show the rest.
+### Completion menu (nvim-cmp)
+
+Active only while the completion popup is open (it appears as you type, e.g.
+file paths):
+
+| Shortcut                | Action                                                             |
+| ----------------------- | ------------------------------------------------------------------ |
+| `Tab` / `Shift`+`Tab`   | Select next / previous item                                        |
+| `Ctrl`+`n` / `Ctrl`+`p` | Select next / previous item                                        |
+| `Ctrl`+`Space`          | Open the completion menu                                           |
+| `Enter`                 | Confirm the highlighted item (no auto-select; otherwise a newline) |
+| `Ctrl`+`e`              | Dismiss the menu                                                   |
+
+LSP covers Nix, Lua, Python and Terraform (the work box adds C# and Helm).
+Files are formatted on save (conform-nvim). `:Git` opens fugitive; gitsigns
+shows gutter signs. which-key pops up after `<leader>` to show the rest.
 
 ---
 

lyrathorpe/home/editor.nix

@@ -89,6 +89,18 @@
         enable = true;
         autoEnableSources = true;
         settings = {
+          # nvim-cmp ships no default keymaps; without these the menu shows but
+          # nothing accepts it. confirm uses select=false so a bare <CR> stays a
+          # newline unless an entry is explicitly highlighted.
+          mapping = {
+            "<C-n>" = "cmp.mapping.select_next_item()";
+            "<C-p>" = "cmp.mapping.select_prev_item()";
+            "<Tab>" = "cmp.mapping.select_next_item()";
+            "<S-Tab>" = "cmp.mapping.select_prev_item()";
+            "<CR>" = "cmp.mapping.confirm({ select = false })";
+            "<C-Space>" = "cmp.mapping.complete()";
+            "<C-e>" = "cmp.mapping.abort()";
+          };
           snippet.expand = "function(args) require('luasnip').lsp_expand(args.body) end";
           sources = [
             { name = "nvim_lsp"; }