Fixes Issue #55: Unicode string truncation causes panics with non-ASCII input
Previously, code used byte-index slicing (`&s[..n]`) which panics when the
slice boundary falls in the middle of a multi-byte UTF-8 character (emoji,
CJK, accented characters).
Changes:
- Added `truncate_with_ellipsis()` helper in `src/util.rs` that uses
`char_indices()` to find safe character boundaries
- Replaced 2 unsafe truncations in `src/channels/mod.rs` with the safe helper
- Added 12 comprehensive tests covering emoji, CJK, accented chars, and edge cases
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
