Add keep-awake option while an Atlas agent is running

[uzayer]

Add a user setting that prevents the computer from sleeping while an Atlas agent has an active run, matching the Codex desktop app’s keep-awake behavior.

Acceptance criteria:

• The setting is discoverable in Settings and defaults to the chosen product-default behavior.
• Atlas acquires the system wake lock only while one or more agent turns are actively running, and releases it reliably on completion, cancellation, error, app shutdown, and setting disable.
• It does not prevent display sleep unnecessarily if the platform supports a narrower “idle sleep” assertion.
• The UI clearly describes what will remain awake and when.