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/dependencies.go

@@ -224,6 +224,7 @@ func (s *SQLiteStorage) GetDependenciesWithMetadata(ctx context.Context, issueID
 		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.deleted_at, i.deleted_by, i.delete_reason, i.original_type,
 		       d.type
 		FROM issues i
 		JOIN dependencies d ON i.id = d.depends_on_id
@@ -244,6 +245,7 @@ func (s *SQLiteStorage) GetDependentsWithMetadata(ctx context.Context, issueID s
 		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.deleted_at, i.deleted_by, i.delete_reason, i.original_type,
 		       d.type
 		FROM issues i
 		JOIN dependencies d ON i.id = d.issue_id
@@ -689,12 +691,17 @@ func (s *SQLiteStorage) scanIssues(ctx context.Context, rows *sql.Rows) ([]*type
 		var externalRef sql.NullString
 		var sourceRepo sql.NullString
 		var closeReason sql.NullString
+		var deletedAt sql.NullTime
+		var deletedBy sql.NullString
+		var deleteReason sql.NullString
+		var originalType sql.NullString
 
 		err := rows.Scan(
 			&issue.ID, &contentHash, &issue.Title, &issue.Description, &issue.Design,
 			&issue.AcceptanceCriteria, &issue.Notes, &issue.Status,
 			&issue.Priority, &issue.IssueType, &assignee, &estimatedMinutes,
 			&issue.CreatedAt, &issue.UpdatedAt, &closedAt, &externalRef, &sourceRepo, &closeReason,
+			&deletedAt, &deletedBy, &deleteReason, &originalType,
 		)
 		if err != nil {
 			return nil, fmt.Errorf("failed to scan issue: %w", err)
@@ -722,6 +729,18 @@ func (s *SQLiteStorage) scanIssues(ctx context.Context, rows *sql.Rows) ([]*type
 		if closeReason.Valid {
 			issue.CloseReason = closeReason.String
 		}
+		if deletedAt.Valid {
+			issue.DeletedAt = &deletedAt.Time
+		}
+		if deletedBy.Valid {
+			issue.DeletedBy = deletedBy.String
+		}
+		if deleteReason.Valid {
+			issue.DeleteReason = deleteReason.String
+		}
+		if originalType.Valid {
+			issue.OriginalType = originalType.String
+		}
 
 		issues = append(issues, &issue)
 		issueIDs = append(issueIDs, issue.ID)
@@ -754,6 +773,10 @@ func (s *SQLiteStorage) scanIssuesWithDependencyType(ctx context.Context, rows *
 		var assignee sql.NullString
 		var externalRef sql.NullString
 		var sourceRepo sql.NullString
+		var deletedAt sql.NullTime
+		var deletedBy sql.NullString
+		var deleteReason sql.NullString
+		var originalType sql.NullString
 		var depType types.DependencyType
 
 		err := rows.Scan(
@@ -761,6 +784,7 @@ func (s *SQLiteStorage) scanIssuesWithDependencyType(ctx context.Context, rows *
 			&issue.AcceptanceCriteria, &issue.Notes, &issue.Status,
 			&issue.Priority, &issue.IssueType, &assignee, &estimatedMinutes,
 			&issue.CreatedAt, &issue.UpdatedAt, &closedAt, &externalRef, &sourceRepo,
+			&deletedAt, &deletedBy, &deleteReason, &originalType,
 			&depType,
 		)
 		if err != nil {
@@ -786,6 +810,18 @@ func (s *SQLiteStorage) scanIssuesWithDependencyType(ctx context.Context, rows *
 		if sourceRepo.Valid {
 			issue.SourceRepo = sourceRepo.String
 		}
+		if deletedAt.Valid {
+			issue.DeletedAt = &deletedAt.Time
+		}
+		if deletedBy.Valid {
+			issue.DeletedBy = deletedBy.String
+		}
+		if deleteReason.Valid {
+			issue.DeleteReason = deleteReason.String
+		}
+		if originalType.Valid {
+			issue.OriginalType = originalType.String
+		}
 
 		// Fetch labels for this issue
 		labels, err := s.GetLabels(ctx, issue.ID)