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/types.go

@@ -254,3 +254,57 @@ func DefaultAccountsConfigDir() string {
 	home, _ := os.UserHomeDir()
 	return home + "/.claude-accounts"
 }
+
+// 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
+
+	// Lists are static mailing lists. Messages are fanned out to all recipients.
+	// Each recipient gets their own copy of the message.
+	// Example: {"oncall": ["mayor/", "gastown/witness"]}
+	Lists map[string][]string `json:"lists,omitempty"`
+
+	// Queues are shared work queues. Only one copy exists; workers claim messages.
+	// Messages sit in the queue until explicitly claimed by a worker.
+	// Example: {"work/gastown": ["gastown/polecats/*"]}
+	Queues map[string]QueueConfig `json:"queues,omitempty"`
+
+	// Announces are bulletin boards. One copy exists; anyone can read, no claiming.
+	// Used for broadcast announcements that don't need acknowledgment.
+	// Example: {"alerts": {"readers": ["@town"]}}
+	Announces map[string]AnnounceConfig `json:"announces,omitempty"`
+}
+
+// QueueConfig represents a work queue configuration.
+type QueueConfig struct {
+	// Workers lists addresses eligible to claim from this queue.
+	// Supports wildcards: "gastown/polecats/*" matches all polecats in gastown.
+	Workers []string `json:"workers"`
+
+	// MaxClaims is the maximum number of concurrent claims (0 = unlimited).
+	MaxClaims int `json:"max_claims,omitempty"`
+}
+
+// AnnounceConfig represents a bulletin board configuration.
+type AnnounceConfig struct {
+	// Readers lists addresses eligible to read from this announce channel.
+	// Supports @group syntax: "@town", "@rig/gastown", "@witnesses".
+	Readers []string `json:"readers"`
+
+	// RetainCount is the number of messages to retain (0 = unlimited).
+	RetainCount int `json:"retain_count,omitempty"`
+}
+
+// CurrentMessagingVersion is the current schema version for MessagingConfig.
+const CurrentMessagingVersion = 1
+
+// NewMessagingConfig creates a new MessagingConfig with defaults.
+func NewMessagingConfig() *MessagingConfig {
+	return &MessagingConfig{
+		Version:   CurrentMessagingVersion,
+		Lists:     make(map[string][]string),
+		Queues:    make(map[string]QueueConfig),
+		Announces: make(map[string]AnnounceConfig),
+	}
+}