feat(bd-1pj6): Add custom status states via config

Users can now define custom status states for multi-step pipelines using:
  bd config set status.custom "awaiting_review,awaiting_testing,awaiting_docs"

Changes:
- Add Status.IsValidWithCustom() method for custom status validation
- Add Issue.ValidateWithCustomStatuses() method
- Add GetCustomStatuses() method to storage interface
- Update CreateIssue/UpdateIssue to support custom statuses
- Add comprehensive tests for custom status functionality
- Update config command help text with custom status documentation

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

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

internal/storage/storage.go

@@ -146,6 +146,7 @@ type Storage interface {
 	GetConfig(ctx context.Context, key string) (string, error)
 	GetAllConfig(ctx context.Context) (map[string]string, error)
 	DeleteConfig(ctx context.Context, key string) error
+	GetCustomStatuses(ctx context.Context) ([]string, error) // Custom status states from status.custom config
 
 	// Metadata (for internal state like import hashes)
 	SetMetadata(ctx context.Context, key, value string) error