Task system event/notification layer

Add an event/notification layer to the task system so it becomes the single source of truth for all work notifications.

What

• Add a task_events table (timestamp, task_id, old_status, new_status, agent/user)
• When tasks transition state, emit an event to this table
• Ava checks task inbox on heartbeats for open loops (Review, NeedsHelp)
• This replaces the pi notification sidecar (webhooks, pi_rpc.py, /pi/notifications)

Why

Consolidating around Pipeline + task system as the sole orchestration layer. Pi agents' only unique value was notifications back to Ava, which this subsumes.

Deprecate

• pi_rpc.py webhook notification endpoint
• /pi/notifications route
• Pi notification sidecar infrastructure

Architecture

Pipeline → reads tasks, checks budget, dispatches to agentd Task system → single source of truth for work + notifications/events agentd → dumb process supervisor Ava → checks task inbox on heartbeats