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/validators.go

@@ -16,10 +16,15 @@ func validatePriority(value interface{}) error {
 	return nil
 }
 
-// validateStatus validates a status value
+// validateStatus validates a status value (built-in statuses only)
 func validateStatus(value interface{}) error {
+	return validateStatusWithCustom(value, nil)
+}
+
+// validateStatusWithCustom validates a status value, allowing custom statuses.
+func validateStatusWithCustom(value interface{}, customStatuses []string) error {
 	if status, ok := value.(string); ok {
-		if !types.Status(status).IsValid() {
+		if !types.Status(status).IsValidWithCustom(customStatuses) {
 			return fmt.Errorf("invalid status: %s", status)
 		}
 	}
@@ -65,8 +70,18 @@ var fieldValidators = map[string]func(interface{}) error{
 	"estimated_minutes": validateEstimatedMinutes,
 }
 
-// validateFieldUpdate validates a field update value
+// validateFieldUpdate validates a field update value (built-in statuses only)
 func validateFieldUpdate(key string, value interface{}) error {
+	return validateFieldUpdateWithCustomStatuses(key, value, nil)
+}
+
+// validateFieldUpdateWithCustomStatuses validates a field update value,
+// allowing custom statuses for status field validation.
+func validateFieldUpdateWithCustomStatuses(key string, value interface{}, customStatuses []string) error {
+	// Special handling for status field to support custom statuses
+	if key == "status" {
+		return validateStatusWithCustom(value, customStatuses)
+	}
 	if validator, ok := fieldValidators[key]; ok {
 		return validator(value)
 	}