Removed dead and inert code from Chat Mode's context manager that shipped without a consumer: the unused ``build_prompt_context()`` and ``_get_recent_messages()`` helpers, the ``conversation_history`` and ``accumulated_sources`` keys in the research-context dict (nothing downstream read them), and the cross-turn source-count tracking (``_extract_sources_from_history``, the ``source_count`` field, and ``update_accumulated_context``'s ``source_count_delta`` parameter) — which never functioned because the producer was never called with sources. The ``chat.max_context_messages`` setting is removed with it, since it only fed the now-deleted recent-message inclusion path. ``chat.max_findings_to_include`` (which still controls how many prior findings carry into a follow-up) is unchanged.
