Address file growth concern from #711 review: logical truncation via
skip offset is fast but leaves dead lines on disk indefinitely.
Compact() rewrites the JSONL file keeping only active messages, using
the same temp+rename pattern for crash safety. No-op when skip == 0.
The caller (lifecycle manager or agent loop) decides when to trigger
compaction — e.g. when skipped lines exceed active lines.
Cover all Store interface methods plus edge cases:
- Basic roundtrip, ordering, empty session, tool calls
- Logical truncation (keep last N, keep zero, keep more than exist)
- SetHistory replacing all + resetting skip offset
- Crash recovery with partial JSON lines
- Persistence across store instances
- Concurrent add+read (10 goroutines x 20 msgs)
- Simulated #704 race (summarizer vs main loop)
- Benchmarks for AddMessage and GetHistory (100/1000 msgs)
xiaoen