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.

pnpm-lock.yaml

@@ -41,7 +41,7 @@ importers:
         version: 2.2.7
       '@vitest/coverage-v8':
         specifier: 3.0.4
-        version: 3.0.4(vitest@3.0.4(@types/debug@4.1.12)(@types/node@22.10.10)(@vitest/ui@3.0.4)(jiti@2.4.2)(lightningcss@1.29.1)(terser@5.37.0)(yaml@2.7.0))
+        version: 3.0.4(vitest@3.0.4)
       '@vitest/ui':
         specifier: ^3.0.4
         version: 3.0.4(vitest@3.0.4)
@@ -221,6 +221,9 @@ importers:
       '@tonaljs/tonal':
         specifier: ^4.10.0
         version: 4.10.0
+      codemirror-helix:
+        specifier: ^0.5.0
+        version: 0.5.0(@codemirror/commands@6.8.0)(@codemirror/language@6.10.8)(@codemirror/search@6.5.8)(@codemirror/state@6.5.1)(@codemirror/view@6.36.2)
       nanostores:
         specifier: ^0.11.3
         version: 0.11.3
@@ -3536,6 +3539,15 @@ packages:
     resolution: {integrity: sha512-FMabTRlc5t5zjdenF6mS0MBeFZm0XqHqeOkcskKFb/LYCcRQ5fVgLOHVc4Lq9CqABd9zhjwPjMBCJvMCziSVtA==}
     engines: {node: ^14.17.0 || ^16.13.0 || >=18.0.0}
 
+  codemirror-helix@0.5.0:
+    resolution: {integrity: sha512-hI56hf9VGz53H1YvL6H1GC7HtP6te8vX+MsIHaE9J7Q3PQ6KFapKtIRg6lqSH898ikHWpMCPu42r6HJN0IfVLA==}
+    peerDependencies:
+      '@codemirror/commands': ^6.0.0
+      '@codemirror/language': ^6.0.0
+      '@codemirror/search': ^6.0.0
+      '@codemirror/state': ^6.0.0
+      '@codemirror/view': ^6.0.0
+
   collapse-white-space@2.1.0:
     resolution: {integrity: sha512-loKTxY1zCOuG4j9f6EPnuyyYkf58RnhhWTvRoZEokgB+WbdXehfjFviyOVYkqzEWz1Q5kRiZdBYS5SwxbQYwzw==}
 
@@ -10554,7 +10566,7 @@ snapshots:
     transitivePeerDependencies:
       - supports-color
 
-  '@vitest/coverage-v8@3.0.4(vitest@3.0.4(@types/debug@4.1.12)(@types/node@22.10.10)(@vitest/ui@3.0.4)(jiti@2.4.2)(lightningcss@1.29.1)(terser@5.37.0)(yaml@2.7.0))':
+  '@vitest/coverage-v8@3.0.4(vitest@3.0.4)':
     dependencies:
       '@ampproject/remapping': 2.3.0
       '@bcoe/v8-coverage': 1.0.2
@@ -11254,6 +11266,14 @@ snapshots:
 
   cmd-shim@6.0.3: {}
 
+  codemirror-helix@0.5.0(@codemirror/commands@6.8.0)(@codemirror/language@6.10.8)(@codemirror/search@6.5.8)(@codemirror/state@6.5.1)(@codemirror/view@6.36.2):
+    dependencies:
+      '@codemirror/commands': 6.8.0
+      '@codemirror/language': 6.10.8
+      '@codemirror/search': 6.5.8
+      '@codemirror/state': 6.5.1
+      '@codemirror/view': 6.36.2
+
   collapse-white-space@2.1.0: {}
 
   color-convert@2.0.1: