* Add multi-message sending via split marker
* Add marker and length split integration tests
Tests that SplitByMarker and SplitMessage work together correctly, and
that code block boundaries are preserved during marker splitting.
* Simplify message chunking logic in channel worker
Extract splitByLength helper function and remove goto-based control
flow.
The logic now flows more naturally - try marker splitting first, then
fall
back to length-based splitting.
* Update multi-message output instructions in agent context
* Add split_on_marker to config defaults
* Add split_on_marker config option
* Rename 'Multi-Message Sending' setting to 'Chatty Mode'
* Add SplitOnMarker config option
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