Add nudge_channels to MessagingConfig schema (gt-3shmx)

- Add NudgeChannels field to MessagingConfig struct in types.go
- Initialize NudgeChannels map in NewMessagingConfig()
- Add validation in validateMessagingConfig(): channel names must be
  non-empty and each channel must have at least one recipient
- Add tests for valid nudge channels and empty recipient validation

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

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

internal/config/types.go

@@ -278,6 +278,11 @@ type MessagingConfig struct {
 	// Used for broadcast announcements that don't need acknowledgment.
 	// Example: {"alerts": {"readers": ["@town"]}}
 	Announces map[string]AnnounceConfig `json:"announces,omitempty"`
+
+	// NudgeChannels are named groups for real-time nudge fan-out.
+	// Like mailing lists but for tmux send-keys instead of durable mail.
+	// Example: {"workers": ["gastown/polecats/*", "gastown/crew/*"], "witnesses": ["*/witness"]}
+	NudgeChannels map[string][]string `json:"nudge_channels,omitempty"`
 }
 
 // QueueConfig represents a work queue configuration.
@@ -306,10 +311,11 @@ 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),
-		Announces: make(map[string]AnnounceConfig),
+		Type:          "messaging",
+		Version:       CurrentMessagingVersion,
+		Lists:         make(map[string][]string),
+		Queues:        make(map[string]QueueConfig),
+		Announces:     make(map[string]AnnounceConfig),
+		NudgeChannels: make(map[string][]string),
 	}
 }