web_search now registers each result as a WEB_RESULT (locator {url}) and
renders a <web_results> block of <document view="excerpt"> [n] passages,
returning Command(update={messages, citation_registry}) like
search_knowledge_base. Collapse the duplicate research-subagent web_search
into the shared tool and teach the prompts to cite web hits with [n].
Add unit tests for role-specific turn extraction in the resolver and for
the transcript renderer: full rendering within budget, dropping oldest
turns with a marker, partial-tail fill of an overflowing turn, and
multi-chat tagging.
