feat: add mail system and CLI commands

- internal/mail: Message types with priority support
- internal/mail: Mailbox JSONL operations (list, get, append, delete)
- internal/mail: Router for address resolution and delivery
- gt mail send: Send messages to agents
- gt mail inbox: List messages (--unread, --json)
- gt mail read: Read and mark messages as read
- Address formats: mayor/, rig/, rig/polecat, rig/refinery
- High priority messages trigger tmux notification
- Auto-detect sender from GT_RIG/GT_POLECAT env vars

Closes gt-u1j.6, gt-u1j.12

Generated with Claude Code

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

internal/mail/types.go

@@ -0,0 +1,67 @@
+// Package mail provides JSONL-based messaging for agent communication.
+package mail
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"time"
+)
+
+// Priority levels for messages.
+type Priority string
+
+const (
+	// PriorityNormal is the default priority.
+	PriorityNormal Priority = "normal"
+
+	// PriorityHigh indicates an urgent message.
+	PriorityHigh Priority = "high"
+)
+
+// Message represents a mail message between agents.
+type Message struct {
+	// ID is a unique message identifier.
+	ID string `json:"id"`
+
+	// From is the sender address (e.g., "gastown/Toast" or "mayor/").
+	From string `json:"from"`
+
+	// To is the recipient address.
+	To string `json:"to"`
+
+	// Subject is a brief summary.
+	Subject string `json:"subject"`
+
+	// Body is the full message content.
+	Body string `json:"body"`
+
+	// Timestamp is when the message was sent.
+	Timestamp time.Time `json:"timestamp"`
+
+	// Read indicates if the message has been read.
+	Read bool `json:"read"`
+
+	// Priority is the message priority.
+	Priority Priority `json:"priority"`
+}
+
+// NewMessage creates a new message with a generated ID.
+func NewMessage(from, to, subject, body string) *Message {
+	return &Message{
+		ID:        generateID(),
+		From:      from,
+		To:        to,
+		Subject:   subject,
+		Body:      body,
+		Timestamp: time.Now(),
+		Read:      false,
+		Priority:  PriorityNormal,
+	}
+}
+
+// generateID creates a random message ID.
+func generateID() string {
+	b := make([]byte, 8)
+	rand.Read(b)
+	return "msg-" + hex.EncodeToString(b)
+}