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/migrations/018_tombstone_columns.go

@@ -0,0 +1,47 @@
+package migrations
+
+import (
+	"database/sql"
+	"fmt"
+)
+
+// MigrateTombstoneColumns adds tombstone support columns to the issues table.
+// These columns support inline soft-delete (bd-vw8) replacing deletions.jsonl:
+// - deleted_at: when the issue was deleted
+// - deleted_by: who deleted the issue
+// - delete_reason: why the issue was deleted
+// - original_type: the issue type before deletion (for tombstones)
+func MigrateTombstoneColumns(db *sql.DB) error {
+	columns := []struct {
+		name         string
+		definition   string
+	}{
+		{"deleted_at", "TEXT"},
+		{"deleted_by", "TEXT DEFAULT ''"},
+		{"delete_reason", "TEXT DEFAULT ''"},
+		{"original_type", "TEXT DEFAULT ''"},
+	}
+
+	for _, col := range columns {
+		var columnExists bool
+		err := db.QueryRow(`
+			SELECT COUNT(*) > 0
+			FROM pragma_table_info('issues')
+			WHERE name = ?
+		`, col.name).Scan(&columnExists)
+		if err != nil {
+			return fmt.Errorf("failed to check %s column: %w", col.name, err)
+		}
+
+		if columnExists {
+			continue
+		}
+
+		_, err = db.Exec(fmt.Sprintf(`ALTER TABLE issues ADD COLUMN %s %s`, col.name, col.definition))
+		if err != nil {
+			return fmt.Errorf("failed to add %s column: %w", col.name, err)
+		}
+	}
+
+	return nil
+}