fix(types): add tombstone validation and block direct status update (bd-md2, bd-y68)

- Add validation in ValidateWithCustomStatuses() requiring deleted_at for tombstones
- Add validation that non-tombstones cannot have deleted_at set
- Block direct status update to tombstone in validateStatusWithCustom()
- Users must use 'bd delete' instead of 'bd update --status=tombstone'

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/types/types.go

@@ -112,6 +112,13 @@ func (i *Issue) ValidateWithCustomStatuses(customStatuses []string) error {
 	if i.Status != StatusClosed && i.ClosedAt != nil {
 		return fmt.Errorf("non-closed issues cannot have closed_at timestamp")
 	}
+	// Enforce tombstone invariants (bd-md2): deleted_at must be set for tombstones, and only for tombstones
+	if i.Status == StatusTombstone && i.DeletedAt == nil {
+		return fmt.Errorf("tombstone issues must have deleted_at timestamp")
+	}
+	if i.Status != StatusTombstone && i.DeletedAt != nil {
+		return fmt.Errorf("non-tombstone issues cannot have deleted_at timestamp")
+	}
 	return nil
 }