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/types/types.go

@@ -22,6 +22,7 @@ type Issue struct {
 	CreatedAt          time.Time      `json:"created_at"`
 	UpdatedAt          time.Time      `json:"updated_at"`
 	ClosedAt           *time.Time     `json:"closed_at,omitempty"`
+	ExternalRef        *string        `json:"external_ref,omitempty"` // e.g., "gh-9", "jira-ABC"
 	Dependencies       []*Dependency  `json:"dependencies,omitempty"` // Populated only for export/import
 }