[FEATURE] Scheduled tasks: auto-archive sessions with no actionable output

Problem

Scheduled tasks that fire on a cron but find no work to do (e.g., "No pending tasks in the Claude-Tasks queue. Exiting silently.") still create full session entries in the Cowork sidebar. Over a single day, this produces significant clutter — five identical "Email taskbox" entries that each say nothing happened.

!Screenshot showing 5 empty Email taskbox sessions in the sidebar

Proposed Solution

Add a suppress_if_empty or auto_archive option to the scheduled task definition (via create_scheduled_task / update_scheduled_task). When enabled:

• If the task session produces no meaningful output (or explicitly signals "nothing to do"), the session is automatically archived or never surfaces in the sidebar at all.
• Tasks that do produce output behave as they do today.

Alternatives Considered

• Manual archive: The context menu already has "Archive," but this defeats the purpose of hands-off automation.
• Consolidating tasks: Merging multiple scheduled tasks into one reduces entries but adds prompt complexity and makes individual task management harder.

Environment

• Cowork desktop app (macOS)
• Multiple scheduled tasks on weekday cron (email triage, Slack triage, chat triage, etc.)