session uses .jsonl but compression is zstd

undohaven doctor. Discussion #3057 quotes the startup error: session artifact uses .jsonl, but this backend is configured for compression "zstd". Move the decompressed copy out of the session directory, or restore the sessions scope from a snapshot.

会话是 .jsonl，但压缩模式是 zstd。讨论 #3057 的启动错误是：会话产物用了 .jsonl，但后端配置的压缩是 zstd。把解压副本移出会话目录，或从快照恢复 sessions。