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>

cmd/bd/import.go

@@ -222,6 +222,13 @@ Behavior:
 						updates["estimated_minutes"] = nil
 					}
 				}
+				if _, ok := rawData["external_ref"]; ok {
+					if issue.ExternalRef != nil {
+						updates["external_ref"] = *issue.ExternalRef
+					} else {
+						updates["external_ref"] = nil
+					}
+				}
 
 				if err := store.UpdateIssue(ctx, issue.ID, updates, "import"); err != nil {
 					fmt.Fprintf(os.Stderr, "Error updating issue %s: %v\n", issue.ID, err)