Add fallback in readSessionMessages() to locate subagent transcript files under <project>/<session>/subagents/agent-<id>.jsonl when the sessionId is actually an agentId. Extract entryToSessionMessage and readMessagesFromLines helpers for reuse between main and subagent transcripts.
Also route 'system' messages through the 'message' event in sessionMessageRouter so subscribers can observe them.
Signed-off-by: 林凯90331 <90331@sangfor.com>
Co-authored-by: CoStrict <zgsm@sangfor.com.cn>