Add expandQueue() to resolve queue workers from messaging.json (gt-xkbze)

Add queue expansion to internal/mail/router.go following the expandList() pattern:
- Add ErrUnknownQueue error for unknown queue names
- Add expandQueue() method to look up QueueConfig from messaging.json
- Add TestExpandQueue and TestExpandQueueNoTownRoot tests

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

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

internal/mail/router.go

@@ -17,6 +17,9 @@ import (
 // ErrUnknownList indicates a mailing list name was not found in configuration.
 var ErrUnknownList = errors.New("unknown mailing list")
 
+// ErrUnknownQueue indicates a queue name was not found in configuration.
+var ErrUnknownQueue = errors.New("unknown queue")
+
 // Router handles message delivery via beads.
 // It routes messages to the correct beads database based on address:
 // - Town-level (mayor/, deacon/) -> {townRoot}/.beads
@@ -96,6 +99,28 @@ func (r *Router) expandList(listName string) ([]string, error) {
 	return recipients, nil
 }
 
+// expandQueue returns the QueueConfig for a queue name.
+// Returns ErrUnknownQueue if the queue is not found.
+func (r *Router) expandQueue(queueName string) (*config.QueueConfig, error) {
+	// Load messaging config from town root
+	if r.townRoot == "" {
+		return nil, fmt.Errorf("%w: %s (no town root)", ErrUnknownQueue, queueName)
+	}
+
+	configPath := config.MessagingConfigPath(r.townRoot)
+	cfg, err := config.LoadMessagingConfig(configPath)
+	if err != nil {
+		return nil, fmt.Errorf("loading messaging config: %w", err)
+	}
+
+	queueCfg, ok := cfg.Queues[queueName]
+	if !ok {
+		return nil, fmt.Errorf("%w: %s", ErrUnknownQueue, queueName)
+	}
+
+	return &queueCfg, nil
+}
+
 // detectTownRoot finds the town root by looking for mayor/town.json.
 func detectTownRoot(startDir string) string {
 	dir := startDir