Persist session IDs across thread resume

[jif-oai]

Summary

A cold-resumed subagent kept its durable thread ID but could receive a new session ID, splitting one agent tree across multiple sessions after a restart.

Persist the root session ID in every rollout SessionMeta, carry it through thread creation, and restore it before initializing the resumed Session and AgentControl.

Behavior

For a nested agent tree:

root session R
  parent thread P
    child thread C

The child rollout stores:

session_id:       R
parent_thread_id: P
id:               C

After a cold resume, the child still belongs to root session R while its immediate parent remains P. The integration coverage uses distinct values for all three IDs so it catches restoring the session from parent_thread_id.

Legacy rollouts

Previous rollouts have id but no session_id. SessionMetaLine deserialization treats a missing session_id as id, keeping those files readable, listable, and resumable. When a legacy subagent is resumed through its root, that synthesized child ID no longer overrides the inherited root-scoped AgentControl. New rollouts always persist the explicit root session ID.

[jif-oai]

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[jif-oai]

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