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/schema.go

@@ -16,7 +16,8 @@ CREATE TABLE IF NOT EXISTS issues (
     estimated_minutes INTEGER,
     created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
     updated_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
-    closed_at DATETIME
+    closed_at DATETIME,
+    external_ref TEXT
 );
 
 CREATE INDEX IF NOT EXISTS idx_issues_status ON issues(status);