Remaining UTF-8 char boundary panics in channel.rs and signal.rs (incomplete fix from #273)

[chandima]

Environment

• Spacebot Version: v0.3.3
• OS: macOS (Apple Silicon M1, 8GB RAM)

Description

Issue #273 fixed a UTF-8 char boundary panic in cortex_chat.rs, but the same unsafe &s[..200] pattern exists in two other files, causing runtime panics when truncating strings containing multi-byte characters (em dashes, CJK, emoji, etc.).

Panic observed

thread 'tokio-runtime-worker' (14991) panicked at src/agent/channel.rs:2907:45:
byte index 200 is not a char boundary; it is inside '—' (bytes 199..202) of
`Here's today's meetings (America/Phoenix local time, 2026-03-31)...`

This crash prevented Spacebot from delivering a calendar summary response via Slack — the calendar data was fetched successfully but the worker result truncation panicked.

Affected locations

1. src/agent/channel.rs:2834 — branch conclusion truncation for working memory
2. src/agent/channel.rs:2907 — worker result truncation for working memory
3. src/messaging/signal.rs:900 — Signal typing indicator error body truncation

Fix

Replace &s[..200] with char-boundary-safe truncation:

let end = s.floor_char_boundary(200);
&s[..end]

floor_char_boundary is stable since Rust 1.82.

Related

• Runtime panic in cortex_chat.rs:92: byte index not on char boundary when processing CJK characters in memory #273 (closed) — same root cause, fixed only in cortex_chat.rs