feat(security): Implement workspace isolation and task definition hiding during agent execution

### Summary
When running DevOps Bench evaluations, evaluated agents with file system tools (or search capabilities) can scan workspace paths, discover task definitions (`tasks/**/task.yaml`), and read golden outputs directly.

### Scope
1. Execute agent subprocesses inside isolated ephemeral directories without mounting the `tasks/` directory.
2. Prevent direct file discovery of golden manifests and evaluation keys.

Ref: Internal bug b/552484058 (b/533473646)