The per-kind fixture guard claimed no-model scenarios need no session.jsonl, but
the replay path requires one for ALL scenarios: runScenario() passes
`fixtureFile: <dir>/session.jsonl` unconditionally and llm-replay's
loadReplayScript() throws "fixture not found" when it is absent and no override
replaces it. A no-model scenario ships a header-only session.jsonl that derives
to an empty script. The guard + its comment now match that reality, so a future
no-model scenario following them won't fail at subprocess startup.