feat(harness): Sanitize environment variables and breadcrumbs in subprocess execution

[pradeepvrd]

Summary

Shell-capable agents can inspect inherited environment variables (such as KUBECONFIG paths containing task names and layout paths) to backtrace task directories.

Scope

1. Sanitize inherited environment variables passed to agent subprocesses.
2. Obfuscate ephemeral run IDs and file paths.

Ref: Internal bug b/552484094