[codex] add root process id setter

[levivilet]

Summary

• add ProcessExplorer.setRootProcessId for updating the worker root process id
• initialize unknown rootPid as -1
• reuse the resolved root process id after the first refresh instead of re-requesting it

Impact

This avoids repeated ProcessId.getMainProcessId calls after the process explorer has already resolved the root PID, while preserving the existing backend process-listing API.

Validation

• cd packages/process-explorer-worker && npm test