feat(memory): add Compact method for physical JSONL compaction

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.

pkg/memory/store.go

@@ -33,6 +33,10 @@ type Store interface {
 	// SetHistory replaces all messages in a session with the provided history.
 	SetHistory(ctx context.Context, sessionKey string, history []providers.Message) error
 
+	// Compact reclaims storage by physically removing logically truncated
+	// data. Backends that do not accumulate dead data may return nil.
+	Compact(ctx context.Context, sessionKey string) error
+
 	// Close releases any resources held by the store.
 	Close() error
 }