[Bug] Fullscreen TUI mode: Cmd+Left/Right shortcuts don't jump cursor to line start/end

Bug Description
Title: /tui fullscreen: Cmd+Left/Right doesn't jump cursor to line start/end

## Environment

• Claude Code (latest)
• Platform: macOS (Darwin 25.3.0)
• Shell: zsh
• Terminal: [fill in — e.g. iTerm2 / Terminal.app / Ghostty / Warp, plus version]
• TUI mode: /tui fullscreen

## Summary

In fullscreen TUI mode, the macOS-standard Cmd+Left / Cmd+Right hotkeys no longer move the cursor to the beginning / end of the
current line in the prompt input.

## Steps to reproduce

1. Launch Claude Code and run /tui fullscreen.
2. Type a multi-character prompt into the input, e.g. hello world.
3. Place the cursor somewhere in the middle of the text.
4. Press Cmd+Left (expect: cursor jumps to start of line).
5. Press Cmd+Right (expect: cursor jumps to end of line).

## Expected

• Cmd+Left → cursor moves to the beginning of the current line.
• Cmd+Right → cursor moves to the end of the current line.

This matches default macOS text-editing behavior and works in the default (non-fullscreen) TUI mode.

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