feat(backend): c2 callbacks + execute endpoints (sprint 8 M2)

- Add C2Error exception to adapter ABC
- Add promote_to_in_progress() helper to simulation_workflow (pending→in_progress)
- Flesh out MythicAdapter: list_callbacks() (GraphQL query) + create_task() (mutation)
- Expand FakeAdapter to 3 deterministic callbacks; switch task store to per-instance
- Add GET /api/engagements/<id>/c2/callbacks — lists active callbacks via adapter
- Add POST /api/simulations/<id>/c2/execute — issues tasks, stores C2Task rows,
  auto-transitions pending→in_progress, blocks on done (409)
- Both endpoints: SOC=403, 503 no-key, 502 adapter error, sanitized error messages
- Add requests-mock==1.12.1 to requirements.txt
- 42 new tests (342 total, 300 M1 baseline preserved green)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/c2/__init__.py

@@ -2,6 +2,7 @@
 from backend.app.services.c2.adapter import (
     C2Adapter,
     C2Callback,
+    C2Error,
     C2Health,
     C2TaskPage,
     C2TaskStatus,
@@ -12,6 +13,7 @@ from backend.app.services.c2.factory import get_adapter
 __all__ = [
     "C2Adapter",
     "C2Callback",
+    "C2Error",
     "C2Health",
     "C2TaskPage",
     "C2TaskStatus",