Three focused modules used by the upcoming resume-chat orchestrator:
* runtime_context: build_resume_chat_runtime_context assembles the
SurfSenseContextSchema for a resume turn (handles empty mention
lists, since resume requests do not carry fresh @-mentions).
* assistant_shell: persist_resume_assistant_shell writes a fresh
assistant row for the resumed turn so the post-stream finalize
has a target.
* resume_routing: build_resume_routing collects the pending
interrupts across paused subagents and slices the flat list of
ResumeDecision[] into the correct (thread, subagent) buckets so
LangGraph routes each decision back to the right paused tool call.
Add-only; no orchestrator yet (next commit).