feat(mail): add interrupt vs queue delivery semantics (gt-6k8)

Add delivery modes to mail messages:
- queue (default): message stored for periodic checking
- interrupt: inject system-reminder directly into session

New features:
- --interrupt flag on gt mail send for urgent/lifecycle messages
- --quiet flag on gt mail check for silent non-blocking checks
- gt mail wait command to block until mail arrives (with optional timeout)

Interrupt delivery injects a system-reminder via tmux send-keys,
useful for stuck detection, nudges, and urgent notifications.

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

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

internal/mail/router.go

@@ -71,8 +71,14 @@ func (r *Router) Send(msg *Message) error {
 		return fmt.Errorf("sending message: %w", err)
 	}
 
-	// Notify recipient if they have an active session
-	r.notifyRecipient(msg)
+	// Handle delivery based on mode
+	if msg.Delivery == DeliveryInterrupt {
+		// Interrupt: inject system-reminder directly into session
+		r.interruptRecipient(msg)
+	} else {
+		// Queue (default): just notify in status line
+		r.notifyRecipient(msg)
+	}
 
 	return nil
 }
@@ -102,6 +108,39 @@ func (r *Router) notifyRecipient(msg *Message) error {
 	return r.tmux.DisplayMessageDefault(sessionID, notification)
 }
 
+// interruptRecipient injects a system-reminder directly into the session.
+// Uses tmux send-keys to inject text that Claude will see as input.
+// This is disruptive - use for lifecycle events, URGENT messages, or stuck detection.
+func (r *Router) interruptRecipient(msg *Message) error {
+	sessionID := addressToSessionID(msg.To)
+	if sessionID == "" {
+		return nil // Unable to determine session ID
+	}
+
+	// Check if session exists
+	hasSession, err := r.tmux.HasSession(sessionID)
+	if err != nil || !hasSession {
+		return nil // No active session, skip interrupt
+	}
+
+	// Build system-reminder with message content
+	priorityStr := ""
+	if msg.Priority == PriorityUrgent {
+		priorityStr = " [URGENT]"
+	} else if msg.Priority == PriorityHigh {
+		priorityStr = " [HIGH PRIORITY]"
+	}
+
+	reminder := fmt.Sprintf("\n<system-reminder>\n📬 NEW MAIL%s from %s\nSubject: %s\n", priorityStr, msg.From, msg.Subject)
+	if msg.Body != "" {
+		reminder += fmt.Sprintf("\n%s\n", msg.Body)
+	}
+	reminder += "\nRun 'gt mail inbox' to see your messages.\n</system-reminder>\n"
+
+	// Inject via send-keys (don't press Enter, just paste)
+	return r.tmux.SendKeysRaw(sessionID, reminder)
+}
+
 // addressToSessionID converts a mail address to a tmux session ID.
 // Returns empty string if address format is not recognized.
 func addressToSessionID(address string) string {

internal/mail/types.go

@@ -41,6 +41,19 @@ const (
 	TypeReply MessageType = "reply"
 )
 
+// Delivery specifies how a message is delivered to the recipient.
+type Delivery string
+
+const (
+	// DeliveryQueue creates the message in the mailbox for periodic checking.
+	// This is the default delivery mode. Agent checks with `gt mail check`.
+	DeliveryQueue Delivery = "queue"
+
+	// DeliveryInterrupt injects a system-reminder directly into the agent's session.
+	// Use for lifecycle events, URGENT priority, or stuck detection.
+	DeliveryInterrupt Delivery = "interrupt"
+)
+
 // Message represents a mail message between agents.
 // This is the GGT-side representation; it gets translated to/from beads messages.
 type Message struct {
@@ -71,6 +84,10 @@ type Message struct {
 	// Type indicates the message type (task, scavenge, notification, reply).
 	Type MessageType `json:"type"`
 
+	// Delivery specifies how the message is delivered (queue or interrupt).
+	// Queue: agent checks periodically. Interrupt: inject into session.
+	Delivery Delivery `json:"delivery,omitempty"`
+
 	// ThreadID groups related messages into a conversation thread.
 	ThreadID string `json:"thread_id,omitempty"`