CLI panics on UTF-8 multi-byte character boundary when processing Chinese full-width parentheses

Description

Claude Code CLI crashes with a Rust panic when processing text containing Chinese
full-width right parenthesis ） (U+FF09) followed by ASCII left parenthesis (.

## Environment

• Claude Code version: v2.0.75
• OS: Windows
• Platform: win32

## Error Message

thread '' panicked at library\core\src\str\mod.rs:833:21:
byte index 2 is not a char boundary; it is inside '）' (bytes 0..3) of ）(

## Steps to Reproduce

1. Open Claude Code CLI
2. Ask Claude to edit a markdown file containing text like:

• 核心组件（周视图 + 时间槽 + 按钮）(~320 行)
• (Chinese full-width right parenthesis ） immediately followed by ASCII () 3. CLI crashes during the Edit operation

## Root Cause

The full-width ） (U+FF09) is 3 bytes in UTF-8. The Rust code slices at byte index 2, which is mid-character, causing panic.

## Workaround

None found - even adding a space between ） and ( still triggers the crash.