Extract _process_chained_node helper to deduplicate interaction chaining loops

Both QUESTION and MESSAGE response branches in process_interaction contained
nearly identical ~140-line chaining loops. Extract shared logic into a single
_process_chained_node method that handles QUESTION, ACTION/CONDITION/COMPOSITE,
and MESSAGE node types uniformly. Also document the emit_event action type
across chatflow-node-types, node-schemas-reference, and architecture docs.

Author: hardbyte