Add messaging config types and load/save functions (gt-i6jvc)

Add MessagingConfig with:
- Lists: static mailing lists with fan-out delivery
- Queues: shared work queues with claiming
- Announces: bulletin boards for broadcast messages

Includes:
- types.go: MessagingConfig, QueueConfig, AnnounceConfig types
- loader.go: Load/Save/Validate functions, MessagingConfigPath helper
- loader_test.go: Round-trip and validation tests

Created ~/gt/config/messaging.json with example configuration.

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

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

internal/config/loader.go

@@ -565,3 +565,111 @@ func expandPath(path string) string {
 	}
 	return path
 }
+
+// LoadMessagingConfig loads and validates a messaging configuration file.
+func LoadMessagingConfig(path string) (*MessagingConfig, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, fmt.Errorf("%w: %s", ErrNotFound, path)
+		}
+		return nil, fmt.Errorf("reading messaging config: %w", err)
+	}
+
+	var config MessagingConfig
+	if err := json.Unmarshal(data, &config); err != nil {
+		return nil, fmt.Errorf("parsing messaging config: %w", err)
+	}
+
+	if err := validateMessagingConfig(&config); err != nil {
+		return nil, err
+	}
+
+	return &config, nil
+}
+
+// SaveMessagingConfig saves a messaging configuration to a file.
+func SaveMessagingConfig(path string, config *MessagingConfig) error {
+	if err := validateMessagingConfig(config); err != nil {
+		return err
+	}
+
+	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
+		return fmt.Errorf("creating directory: %w", err)
+	}
+
+	data, err := json.MarshalIndent(config, "", "  ")
+	if err != nil {
+		return fmt.Errorf("encoding messaging config: %w", err)
+	}
+
+	if err := os.WriteFile(path, data, 0644); err != nil {
+		return fmt.Errorf("writing messaging config: %w", err)
+	}
+
+	return nil
+}
+
+// validateMessagingConfig validates a MessagingConfig.
+func validateMessagingConfig(c *MessagingConfig) error {
+	if c.Version > CurrentMessagingVersion {
+		return fmt.Errorf("%w: got %d, max supported %d", ErrInvalidVersion, c.Version, CurrentMessagingVersion)
+	}
+
+	// Initialize nil maps
+	if c.Lists == nil {
+		c.Lists = make(map[string][]string)
+	}
+	if c.Queues == nil {
+		c.Queues = make(map[string]QueueConfig)
+	}
+	if c.Announces == nil {
+		c.Announces = make(map[string]AnnounceConfig)
+	}
+
+	// Validate lists have at least one recipient
+	for name, recipients := range c.Lists {
+		if len(recipients) == 0 {
+			return fmt.Errorf("%w: list '%s' has no recipients", ErrMissingField, name)
+		}
+	}
+
+	// Validate queues have at least one worker
+	for name, queue := range c.Queues {
+		if len(queue.Workers) == 0 {
+			return fmt.Errorf("%w: queue '%s' has no workers", ErrMissingField, name)
+		}
+		if queue.MaxClaims < 0 {
+			return fmt.Errorf("queue '%s': max_claims must be non-negative", name)
+		}
+	}
+
+	// Validate announces have at least one reader
+	for name, announce := range c.Announces {
+		if len(announce.Readers) == 0 {
+			return fmt.Errorf("%w: announce '%s' has no readers", ErrMissingField, name)
+		}
+		if announce.RetainCount < 0 {
+			return fmt.Errorf("announce '%s': retain_count must be non-negative", name)
+		}
+	}
+
+	return nil
+}
+
+// MessagingConfigPath returns the standard path for messaging config in a town.
+func MessagingConfigPath(townRoot string) string {
+	return filepath.Join(townRoot, "config", "messaging.json")
+}
+
+// LoadOrCreateMessagingConfig loads the messaging config, creating a default if not found.
+func LoadOrCreateMessagingConfig(path string) (*MessagingConfig, error) {
+	config, err := LoadMessagingConfig(path)
+	if err != nil {
+		if errors.Is(err, ErrNotFound) {
+			return NewMessagingConfig(), nil
+		}
+		return nil, err
+	}
+	return config, nil
+}