Persist session IDs across thread resume (#29327)

## 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:

```text
root session R
  parent thread P
    child thread C
```

The child rollout stores:

```text
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.

codex-rs/cli/src/doctor/thread_inventory.rs

@@ -816,11 +816,13 @@ mod tests {
             };
             std::fs::create_dir_all(&root).expect("rollout dir");
             let path = root.join(format!("rollout-{timestamp}-{thread_id}.jsonl"));
+            let parsed_thread_id = ThreadId::from_string(thread_id).expect("thread id");
             let rollout_line = RolloutLine {
                 timestamp: timestamp.to_string(),
                 item: RolloutItem::SessionMeta(codex_protocol::protocol::SessionMetaLine {
                     meta: codex_protocol::protocol::SessionMeta {
-                        id: ThreadId::from_string(thread_id).expect("thread id"),
+                        session_id: parsed_thread_id.into(),
+                        id: parsed_thread_id,
                         timestamp: timestamp.to_string(),
                         cwd: self.codex_home.path().to_path_buf(),
                         originator: "test".to_string(),