Multiple packages had their own private truncate implementations:
- channels/telegram.go: truncateString (byte-based, no "...")
- channels/dingtalk.go: truncateStringDingTalk (byte-based, no "...")
- voice/transcriber.go: truncateText (byte-based, with "...")
All three are functionally equivalent to the existing utils.Truncate,
which already handles rune-safe truncation and appends "..." correctly.
Replace all private copies with utils.Truncate and delete the dead code.
