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

[pradeepvrd]

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)

[isadominguez314]

/assign @isadominguez314