feat(agent): add delete and clear history for scheduled task runs

[Aarjav Jain (A2rjav)]

Summary

Closes #926

• Adds a Trash2 delete button on each completed/failed run in the Scheduled Tasks results list — running tasks cannot be deleted
• Adds a Clear All History button at the top of the results panel
• Wires onDeleteRun / onClearRuns callbacks from ScheduledTasksPage → ScheduledTaskResults
• Adds clearJobRuns() to useScheduledJobRuns hook (sets scheduledJobRunStorage to [])
• Adds SCHEDULED_TASK_RUN_DELETED_EVENT and SCHEDULED_TASK_RUNS_CLEARED_EVENT analytics events

Test plan

• Create a scheduled task and run it manually a few times
• Verify delete (Trash2) button appears on completed/failed runs but NOT on running runs
• Click delete on a single run → run disappears, others remain
• Refresh page → deleted run does not reappear (removed from storage)
• Click Clear All History → all runs removed, results tab shows empty state
• Refresh page → no runs visible

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[Aarjav Jain (A2rjav)]

I have read the CLA Document and I hereby sign the CLA