Add JSONLStore that persists sessions as .jsonl files (one message per
line) plus .meta.json for summary and truncation offset.
Key design decisions:
- Append-only writes — no full-file rewrites on AddMessage
- Logical truncation via skip offset instead of physical deletion
- Per-session mutex for safe concurrent access
- Crash recovery: malformed trailing lines are silently skipped
- Atomic metadata writes using temp+rename
Zero new dependencies — pure stdlib.
Refs #711
xiaoen