CollaborativeTaskRunner hand-rolled its own step loop instead of
composing runtime/task.py's TaskRunner as design.md D3 requires,
silently dropping Timeline recording, WorldModel wiring,
TaskMetadataStore sync, and the on_task_succeeded/skill-synthesis
hook. Now calls TaskRunner's shared _start_world_view/
_record_step_result helpers for that bookkeeping. Also fixes
pre_observation being reused stale across steps in a multi-step plan
instead of refreshing to the prior step's post-observation.
openspec: multi-agent-collaboration capability, archived change multi-agent-runtime
q792602257