Allow configuring voice mode push-to-talk keybind

Feature Request

The voice mode push-to-talk key is currently hardcoded to Space. It would be great to make this configurable via keybindings.json, similar to how all other keybindings can be customized.

Motivation

• Terminal multiplexer / editor conflicts: Users running Claude Code inside NeoVim terminal, tmux, or other environments may have Space bound to other actions (e.g., leader key in Vim). The hardcoded Space binding can conflict with these workflows.
• Ergonomic preference: Some users would prefer to use modifier keys like Right Cmd, Right Alt, or other keys that are easier to hold while speaking.
• Consistency: Every other keybinding in Claude Code is already configurable through keybindings.json — voice push-to-talk is the exception.

Proposed Solution

Add a voice:record (or similar) action to the keybindings schema so users can remap it:

{
  "context": "Chat",
  "bindings": {
    "meta_right": "voice:record"
  }
}

Current Workaround

None — the Space key is the only way to trigger push-to-talk in voice mode.