Drop startup context when truncating forked rollouts

[jif-oai]

Summary

• Change last-n fork truncation to start at the first fork-turn boundary instead of returning the full rollout when the fork history is shorter than the requested window.
• Add coverage for the startup-prefix case in both rollout truncation tests and agent control spawn behavior.
• Ensure bounded forked children still rebuild context after the cached prefix is truncated.

Testing

• Added unit coverage for truncation behavior when the parent history is under the requested fork-turn limit.
• Added an agent control test covering bounded fork spawn behavior with startup context present.
• Not run (not requested).

[jif-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. You're on a roll.

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