[rollout_trace] Trace sessions and multi-agent edges (openai#18879)

## Summary

Adds the remaining session and multi-agent edge wiring needed to
reconstruct rollout relationships across spawned agents, resumed
sessions, and parent/child message delivery.

## Stack

This is PR 4/5 in the rollout trace stack.

- [openai#18876](openai#18876): Add rollout
trace crate
- [openai#18877](openai#18877): Record core
session rollout traces
- [openai#18878](openai#18878): Trace tool and
code-mode boundaries
- [openai#18879](openai#18879): Trace sessions
and multi-agent edges
- [openai#18880](openai#18880): Add debug trace
reduction command

## Review Notes

This is the stack layer that makes traces useful for multi-threaded
agent workflows. The main invariant is that reconstructed relationships
should come from durable rollout data rather than transient in-memory
manager state wherever possible.

The PR is intentionally small relative to the preceding layers: it uses
the recorder and reducer contracts already established by the stack and
only adds the session/agent relationship events needed by later debug
reduction.

Author: cassirer-openai