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/sqlite/config.go

@@ -3,6 +3,7 @@ package sqlite
 import (
 	"context"
 	"database/sql"
+	"strings"
 )
 
 // SetConfig sets a configuration value
@@ -93,3 +94,37 @@ func (s *SQLiteStorage) GetMetadata(ctx context.Context, key string) (string, er
 	}
 	return value, wrapDBError("get metadata", err)
 }
+
+// CustomStatusConfigKey is the config key for custom status states
+const CustomStatusConfigKey = "status.custom"
+
+// GetCustomStatuses retrieves the list of custom status states from config.
+// Custom statuses are stored as comma-separated values in the "status.custom" config key.
+// Returns an empty slice if no custom statuses are configured.
+func (s *SQLiteStorage) GetCustomStatuses(ctx context.Context) ([]string, error) {
+	value, err := s.GetConfig(ctx, CustomStatusConfigKey)
+	if err != nil {
+		return nil, err
+	}
+	if value == "" {
+		return nil, nil
+	}
+	return parseCustomStatuses(value), nil
+}
+
+// parseCustomStatuses splits a comma-separated string into a slice of trimmed status names.
+// Empty entries are filtered out.
+func parseCustomStatuses(value string) []string {
+	if value == "" {
+		return nil
+	}
+	parts := strings.Split(value, ",")
+	result := make([]string, 0, len(parts))
+	for _, p := range parts {
+		trimmed := strings.TrimSpace(p)
+		if trimmed != "" {
+			result = append(result, trimmed)
+		}
+	}
+	return result
+}