Feature request: Add cursorLineStart/cursorLineEnd actions for Chat input

Summary

When the chat input has a single line and the cursor is in the middle, pressing up/down currently triggers history:previous/history:next. There is no way to rebind them to jump to line start/end because no cursor-positioning actions exist in the keybinding system.

Requested Change

Add chat:cursorLineStart and chat:cursorLineEnd (or equivalent) to the available actions so users can bind them. For example:

{
  "context": "Chat",
  "bindings": {
    "up": "chat:cursorLineStart",
    "down": "chat:cursorLineEnd"
  }
}

Motivation

• Codex CLI already implements this behavior: in a single-line input, up jumps to line start and down jumps to line end. Claude Code users familiar with Codex expect the same.
• cmd+← / cmd+→ currently work as a workaround on macOS, but up/down is more ergonomic for single-line navigation and the keys are otherwise unused (no multi-line to traverse).
• Unbinding up/down via null just makes them inert — they don't fall through to OS-level cursor movement because the TUI framework consumes the event.

Current workaround

cmd+← / cmd+→ on macOS.