fix: auto-persist cleaned history when orphan tool messages detected (#128)

sanitizeHistory now returns dropped count so callers know when orphaned
tool_use/tool_result messages were removed. When orphans are found in
buildMessages, the full session history is sanitized and persisted,
preventing repeated warnings on every request.

- Add SetHistory() to SessionStore interface and both implementations
- Adapt memoryflush caller to new two-return signature
- Change sanitize log level from Warn to Debug

internal/agent/memoryflush.go

@@ -132,7 +132,8 @@ func (l *Loop) runMemoryFlush(ctx context.Context, sessionKey string, settings *
 	if len(recentHistory) > 10 {
 		recentHistory = recentHistory[len(recentHistory)-10:]
 	}
-	messages = append(messages, sanitizeHistory(recentHistory)...)
+	sanitized, _ := sanitizeHistory(recentHistory)
+	messages = append(messages, sanitized...)
 
 	// Flush prompt
 	messages = append(messages, providers.Message{