[Bug] Panic on UTF-8 multibyte character boundary in Japanese text

Description

Claude Code CLI panics when processing Japanese text that contains multibyte characters. The panic occurs when the code attempts to slice a string at a byte index that falls in the middle of a multibyte UTF-8 character.

Error Message

thread '<unnamed>' (2356591) panicked at /rustc/ed61e7d7e242494fb7057f2657300d9e77bb4fcb/library/core/src/str/mod.rs:833:21:
byte index 23 is not a char boundary; it is inside 'ィ' (bytes 21..24) of `情報セキュリティ`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
fatal runtime error: failed to initiate panic, error 5, aborting
[1]    78853 abort      claude

Analysis

The panic occurs because:

1. Rust encodes strings in UTF-8
2. The Japanese character 'ィ' spans 3 bytes (21-24)
3. The code attempts to slice the string at byte index 23, which is in the middle of the character
4. This causes an invalid slice operation and triggers a panic

Environment

• macOS Darwin 25.0.0
• Claude Code CLI (latest version)

Expected Behavior

The CLI should properly handle multibyte UTF-8 characters (Japanese, Chinese, emoji, etc.) and not crash when processing them.

Suggested Fix

Use character-aware string operations like char_indices() instead of direct byte indexing when slicing strings that may contain multibyte characters.