feat: add conversation run lifecycle

[roackb2]

Summary

• promote the existing session run coordinator into a public, transport-neutral core service
• add ordered activity streaming, cancellation, approval resolution, bounded replay, and turn/continue handles
• migrate the control plane to the same implementation and document the programmatic API

Verification

• yarn typecheck
• yarn lint
• yarn build
• yarn test (616 unit tests; integration suite passed)