feat: Add Helix keybindings and Nix development environment

Add support for Helix editor keybindings in the REPL, providing users
with a modern, selection-first modal editing experience as an alternative
to Vim, Emacs, and VSCode keybindings.

Helix Keybindings Integration:

- Add codemirror-helix@^0.5.0 dependency to @strudel/codemirror
- Import and register helix keybindings in keybindings.mjs
- Add 'Helix' option to keybindings selector in Settings UI
- Create comprehensive user documentation at technical-manual/helix.mdx
- Update developer documentation with keybinding integration pattern

The Helix mode provides selection-first editing (select → action) which
differs from Vim's action-motion paradigm, offering an alternative
workflow for modal editing enthusiasts.

Documentation:

- Update CLAUDE.md with Helix support and keybinding addition process
- Create helix.mdx with user-facing documentation and command reference
- Update CHANGELOG.md with December 2025 entry

Testing:

Run `pnpm i` to install new dependencies, then `pnpm dev` to test.
Verify Helix mode works in Settings → Keybindings → Helix.

website/src/repl/components/panel/SettingsTab.jsx

@@ -220,7 +220,7 @@ export function SettingsTab({ started }) {
         <ButtonGroup
           value={keybindings}
           onChange={(keybindings) => settingsMap.setKey('keybindings', keybindings)}
-          items={{ codemirror: 'Codemirror', vim: 'Vim', emacs: 'Emacs', vscode: 'VSCode' }}
+          items={{ codemirror: 'Codemirror', vim: 'Vim', emacs: 'Emacs', helix: 'Helix', vscode: 'VSCode' }}
         ></ButtonGroup>
       </FormItem>
       <FormItem label="Panel Position">