Add optional `parentAgentId` to the session-event envelope for nested sub-agents

Sub-agent session events carry only the innermost `agentId` on the event envelope. When sub-agents nest, a consumer receiving the event stream cannot reconstruct parent-of relationships — there is no field linking a sub-agent to the agent that spawned it.

Today a consumer has to infer the parent indirectly, by correlating each sub-agent's id with the id of the tool call that started it. That works but is fragile and couples consumers to an implementation detail.

**Request:** add an explicit optional `parentAgentId` to the session-event envelope — absent for top-level/root agents, set to the spawning agent's id for nested sub-agents — so parent chains are recoverable directly from the event stream.

issue 中没有指出文件或测试；先定位 session-event envelope 的定义，以及创建嵌套 sub-agent 事件的代码。追踪生成事件的 agent id 如何可用，然后验证 root 事件省略 parentAgentId，嵌套事件包含该字段，并且 consumer 可以从事件流中重建 parent 链。

由索引模型根据 Issue 内容生成。