fix: support delete in --no-db mode (GH#822)

Add CreateTombstone() to MemoryStorage and deleteBatchFallback() to
handle deletion when SQLite is not available. This fixes the error
"tombstone operation not supported by this storage backend" when
using bd delete with --no-db flag.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/memory/memory.go

@@ -474,6 +474,41 @@ func (m *MemoryStorage) CloseIssue(ctx context.Context, id string, reason string
 	}, actor)
 }
 
+// CreateTombstone converts an existing issue to a tombstone record.
+// This is a soft-delete that preserves the issue with status="tombstone".
+func (m *MemoryStorage) CreateTombstone(ctx context.Context, id string, actor string, reason string) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	issue, ok := m.issues[id]
+	if !ok {
+		return fmt.Errorf("issue not found: %s", id)
+	}
+
+	now := time.Now()
+	issue.OriginalType = string(issue.IssueType)
+	issue.Status = types.StatusTombstone
+	issue.DeletedAt = &now
+	issue.DeletedBy = actor
+	issue.DeleteReason = reason
+	issue.UpdatedAt = now
+
+	// Mark as dirty for export
+	m.dirty[id] = true
+
+	// Record tombstone creation event
+	event := &types.Event{
+		IssueID:   id,
+		EventType: "deleted",
+		Actor:     actor,
+		Comment:   &reason,
+		CreatedAt: now,
+	}
+	m.events[id] = append(m.events[id], event)
+
+	return nil
+}
+
 // DeleteIssue permanently deletes an issue and all associated data
 func (m *MemoryStorage) DeleteIssue(ctx context.Context, id string) error {
 	m.mu.Lock()