Reopen a previously completed milestone, marking it as pending again. This also reopens all associated tasks.

Use Cases:

• Undo accidental completion
• Revert when issues are discovered
• Restart milestone workflow

What Happens:

1. Milestone completion is cleared
2. Completion timestamp and user are removed
3. All associated completed tasks are reopened (set back to pending)
4. Progress metrics are updated

Idempotent:
Safe to call on already-pending milestones - returns success without changes.

Example Response:

{
  "id": "milestone-uuid",
  "name": "Inspection Completed",
  "completed": false,
  "completedAt": null,
  "completedByUserId": null,
  "tasksProgressPercentage": 75
}