Fix MessagingConfig code review issues (gt-mrqiz, gt-ngoe6, gt-akc4m)

- Add Type field for schema consistency (gt-mrqiz)
- Fix error message inconsistency: all validation errors now wrap sentinel (gt-ngoe6)
- Add missing tests: wrong type, future version, malformed JSON (gt-akc4m)

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/config/types.go

@@ -258,7 +258,8 @@ func DefaultAccountsConfigDir() string {
 // MessagingConfig represents the messaging configuration (config/messaging.json).
 // This defines mailing lists, work queues, and announcement channels.
 type MessagingConfig struct {
-	Version int `json:"version"` // schema version
+	Type    string `json:"type"`    // "messaging"
+	Version int    `json:"version"` // schema version
 
 	// Lists are static mailing lists. Messages are fanned out to all recipients.
 	// Each recipient gets their own copy of the message.
@@ -302,6 +303,7 @@ const CurrentMessagingVersion = 1
 // NewMessagingConfig creates a new MessagingConfig with defaults.
 func NewMessagingConfig() *MessagingConfig {
 	return &MessagingConfig{
+		Type:      "messaging",
 		Version:   CurrentMessagingVersion,
 		Lists:     make(map[string][]string),
 		Queues:    make(map[string]QueueConfig),