Platform coordinator.py is the single source of truth; the skill
coordinator template (templates/roles/00-coordinator.md + SKILL.md
appendix 00 + body) now derives from it instead of describing a
second, model-played coordinator (U1-U13).
- server: _create_task accepts parent_task_id (404/400 checks,
coordinator-under-coordinator rejected); follow-ups to a running
coordinator return 409 at entry (_create_followup /
_deliver_to_thread) with documented alternatives (U14/U15).
- tests: 8 new coordinator tests (depth limit, lineage, replay,
follow-up 409); appendix sync/provenance/transplant suites green.
- check_appendix.py: 12 chapters match templates/roles/.
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