feat: Add external_ref field for linking to external issue trackers

Add nullable external_ref TEXT field to link bd issues with external
systems like GitHub Issues, Jira, etc. Includes automatic schema
migration for backward compatibility.

Changes:
- Added external_ref column to issues table with feature-based migration
- Updated Issue struct with ExternalRef *string field
- Added --external-ref flag to bd create and bd update commands
- Updated all SQL queries across the codebase to include external_ref:
  - GetIssue, CreateIssue, UpdateIssue, SearchIssues
  - GetDependencies, GetDependents, GetDependencyTree
  - GetReadyWork, GetBlockedIssues, GetIssuesByLabel
- Added external_ref handling in import/export logic
- Follows existing patterns for nullable fields (sql.NullString)

This enables tracking relationships between bd issues and external
systems without requiring changes to existing databases or JSONL files.

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

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

internal/storage/sqlite/labels.go

@@ -108,7 +108,7 @@ func (s *SQLiteStorage) GetIssuesByLabel(ctx context.Context, label string) ([]*
 	rows, err := s.db.QueryContext(ctx, `
 		SELECT i.id, 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.created_at, i.updated_at, i.closed_at, i.external_ref
 		FROM issues i
 		JOIN labels l ON i.id = l.issue_id
 		WHERE l.label = ?