Summary

Implement the user-facing migration experience when ceremony scheduling strategies change between sprints. Strategy changes are locked per-sprint and take effect at the next sprint start.

Design Reference

See Ceremony Scheduling design page -- "Runtime Mutability" section.

Requirements

When a strategy change is pending (detected at sprint boundary):

1. Migration notification: Send a notification to the responsible role (scrum master, department head, or whoever manages the task list) informing them of the strategy change.
2. Warning display: Show a warning that the change takes effect at the next sprint start and may cause initial optimization issues in the first few sprints until the new cadence stabilizes.
3. Reorder prompt: The responsible role may need to reorder or reorganize the backlog for the new system. Surface this as an action item.
4. Velocity window reset: When strategy changes, the velocity rolling-average window resets. Display a "insufficient velocity history for new strategy" notice.

Implementation

• Strategy change detection in CeremonyScheduler.activate_sprint() -- compare previous strategy to new
• Notification via the communication/meeting system (or a dedicated notification channel)
• Warning events: SPRINT_CEREMONY_STRATEGY_CHANGED (already defined in feat: sprint ceremony runtime scheduler with task-driven cadence support #961)
• Dashboard notification component (follow-up from dashboard UI issue)

Dependencies

• feat: sprint ceremony runtime scheduler with task-driven cadence support #961 (CeremonyScheduler, event constants)
• Strategy implementations must exist to test switching between them