[Feature]: Dependency Unblock Notifications (TASK_UNBLOCKED Event & Outbox Dispatch)

[espdesign]

Summary

Automatically detect when prerequisite tasks finish and dispatch a transactional TASK_UNBLOCKED outbox notification to dependent task assignees and watchers, alerting them that work is ready to proceed.

Background & Context

Tech Tree DAGs and prerequisite dependencies are central to dgg-pm. However, when Task A is marked completed, contributors assigned to dependent Task B currently receive no notification that their blocker was resolved. They must periodically check the forum to discover when they can begin.

Proposed Capabilities

1. Unblock Evaluation in TaskService.update_status():
   • When a task transitions to TaskStatus.COMPLETED, query all immediate downstream dependent tasks.
   • For each dependent task, check if all of its prerequisite dependencies are now in TaskStatus.COMPLETED state.
2. Transactional Event Generation:
   • For each newly unblocked task, atomically enqueue an EventType.TASK_UNBLOCKED outbox event with task metadata, assignee ID, and thread snowflake.
3. Notification Delivery:
   • In DiscordNotifier, dispatch an alert via DM / thread ping:

     "🎉 All prerequisite blockers for [INF-2] Frontend UI have completed! You are now clear to begin work."

   • Includes a direct jump link button to open the Task Thread Workspace.

Acceptance Criteria

• Completing a prerequisite task triggers evaluation of downstream dependents.
• TASK_UNBLOCKED event enqueued only when 100% of prerequisites for a task are satisfied.
• Dispatches notifications honoring user preferences (DM, Channel, Both).
• Reopening a task does not result in false positive unblock notifications.
• Unit and integration tests covering outbox event dispatch and notification delivery.

[espdesign]

Moved to milestone Post-Alpha per roadmap reprioritization in #29.