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.
Introduce a backend-agnostic Store interface in pkg/memory/ that maps
one-to-one with the current SessionManager API. Each method is atomic
— no separate Save() call needed.
Refs #711
xiaoen