Remove private reasoning and textual protocol artifacts before content is
handed to a user-facing channel. Real tool calls are structured blocks; a
literal [tool_use: ...] line is therefore always an implementation leak.
Cap on the excerpt embedded by quoted_context_prefix.
Build the [quoted from …] prefix that channel adapters prepend when a
user replies to an earlier message, so terse follow-ups (“why?”, “fix
it”) keep their referent even after the original scrolled out of the
session window. Returns None when the referenced content is empty.