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/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"`