Telegram clients split messages above 4096 chars into multiple updates. A
long '/queue <prompt>' paste arrives as a COMMAND chunk near the limit plus
plain TEXT continuation chunk(s). _handle_command dispatched the command
chunk immediately, so the continuation landed as a separate plain message
that interrupted the running agent instead of being queued.
Near-limit (>= _SPLIT_THRESHOLD) command chunks now route through the same
text-batching pipeline used for split plain-text messages, merging the
continuation before dispatch. Short commands (/stop, /approve, ...) keep the
immediate path and are never delayed.
