feat: unify coordinator semantics (Muse role alignment phase 1)

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/.

cloud/tests/test_skill_appendix_sync.py

@@ -12,7 +12,7 @@ seeded file -- a dropped middle chapter (01..07) would pass CI silently.
 The bidirectional assertion previously discussed on the #209 track is
 unmerged, so main does not have it; this file supplies it on the main
 base (skill text v0.2, 9 appendix chapters 00..08;
-  integrated text is v0.5, 11 appendix chapters 00..10).
+  integrated text is v0.5, 12 appendix chapters 00..11).
 
 What "bidirectional" means here, concretely (all derived from SKILL.md
 itself, no new semantics invented):