context: Collapse diff_traces retirement plan into one atomic task

Merge the progress/heading and orchestration tasks so diff_traces never
syncs invisibly, fold durable context updates into T01's task context
synchronization, and narrow the Phase 3 release gate so
/state.cursors.diffTraces survives until a later compatibility-removal
phase.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Co-authored-by: SCE <sce@crocoder.dev>

Author: davidabram