#2619 feat: run-level --load-trajectory for native agent trajectories

Activate the reserved agent.load_trajectory interface from #2144: load a native trajectory file recorded by a previous run (claude-code .jsonl, codex rollout-*.jsonl) as the agent's session before the first step, which then resumes it instead of starting fresh.

Tested e2e with codex and claude_code, load works fine.

---