feat(types): add tombstone support for inline soft-delete (bd-fbj)

Add tombstone types and schema migration as foundation for the tombstone
epic (bd-vw8) which replaces deletions.jsonl with inline tombstones.

Changes:
- Add tombstone fields to Issue struct: DeletedAt, DeletedBy, DeleteReason, OriginalType
- Add StatusTombstone constant and IsTombstone() helper method
- Update Status.IsValid() to accept tombstone status
- Create migration 018_tombstone_columns.go for new database columns
- Update schema.go with tombstone columns: deleted_at, deleted_by, delete_reason, original_type
- Update all issue insert/update/scan operations across:
  - issues.go (insertIssue, insertIssues)
  - queries.go (GetIssue, GetIssueByExternalRef, SearchIssues)
  - dependencies.go (scanIssues, scanIssuesWithDependencyType)
  - transaction.go (scanIssueRow, GetIssue, SearchIssues)
  - multirepo.go (import operations)
  - ready.go (GetReadyWork, GetStaleIssues)
  - labels.go (GetIssuesByLabel)
- Add test for IsTombstone() helper
- Update migration test to include tombstone columns

Unblocks: bd-olt (TTL logic), bd-3b4 (delete command), bd-0ih (merge updates), bd-dve (import/export)

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

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

internal/storage/sqlite/labels.go

@@ -157,7 +157,8 @@ func (s *SQLiteStorage) GetIssuesByLabel(ctx context.Context, label string) ([]*
 	rows, err := s.db.QueryContext(ctx, `
 		SELECT i.id, i.content_hash, i.title, i.description, i.design, i.acceptance_criteria, i.notes,
 		       i.status, i.priority, i.issue_type, i.assignee, i.estimated_minutes,
-		       i.created_at, i.updated_at, i.closed_at, i.external_ref, i.source_repo, i.close_reason
+		       i.created_at, i.updated_at, i.closed_at, i.external_ref, i.source_repo, i.close_reason,
+		       i.deleted_at, i.deleted_by, i.delete_reason, i.original_type
 		FROM issues i
 		JOIN labels l ON i.id = l.issue_id
 		WHERE l.label = ?