Persist agent system_prompt to task_run metadata for QA workflows

[shfunc]

Note

Medium Risk
Adds system_prompt to eval metadata sent to the backend, which could increase stored data volume and may surface sensitive prompt content if not intended.

Overview
Ensures an eval run’s agent system_prompt is persisted to backend storage by copying it into metadata["agent_config"]["system_prompt"] when building an EvalContext from a task and again just before constructing the enter/exit payload.

This makes QA/workflow tooling able to retrieve the exact system prompt used for a task_run, even when the caller didn’t explicitly include it in metadata.

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