Add closed_at timestamp tracking to issues

- Add closed_at field to Issue type with JSON marshaling
- Implement closed_at timestamp in SQLite storage layer
- Update import/export to handle closed_at field
- Add comprehensive tests for closed_at functionality
- Maintain backward compatibility with existing databases

Amp-Thread-ID: https://ampcode.com/threads/T-f3a7799b-f91e-4432-a690-aae0aed364b3
Co-authored-by: Amp <amp@ampcode.com>

internal/types/types.go

@@ -46,6 +46,13 @@ func (i *Issue) Validate() error {
 	if i.EstimatedMinutes != nil && *i.EstimatedMinutes < 0 {
 		return fmt.Errorf("estimated_minutes cannot be negative")
 	}
+	// Enforce closed_at invariant: closed_at should be set if and only if status is closed
+	if i.Status == StatusClosed && i.ClosedAt == nil {
+		return fmt.Errorf("closed issues must have closed_at timestamp")
+	}
+	if i.Status != StatusClosed && i.ClosedAt != nil {
+		return fmt.Errorf("non-closed issues cannot have closed_at timestamp")
+	}
 	return nil
 }