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/mailbox.go

@@ -0,0 +1,222 @@
+package mail
+
+import (
+	"bufio"
+	"encoding/json"
+	"errors"
+	"os"
+	"path/filepath"
+	"sort"
+)
+
+// Common errors
+var (
+	ErrMessageNotFound = errors.New("message not found")
+	ErrEmptyInbox      = errors.New("inbox is empty")
+)
+
+// Mailbox manages a JSONL-based inbox.
+type Mailbox struct {
+	path string
+}
+
+// NewMailbox creates a mailbox at the given path.
+func NewMailbox(path string) *Mailbox {
+	return &Mailbox{path: path}
+}
+
+// Path returns the mailbox file path.
+func (m *Mailbox) Path() string {
+	return m.path
+}
+
+// List returns all messages in the mailbox.
+func (m *Mailbox) List() ([]*Message, error) {
+	file, err := os.Open(m.path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil
+		}
+		return nil, err
+	}
+	defer file.Close()
+
+	var messages []*Message
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		line := scanner.Text()
+		if line == "" {
+			continue
+		}
+
+		var msg Message
+		if err := json.Unmarshal([]byte(line), &msg); err != nil {
+			continue // Skip malformed lines
+		}
+		messages = append(messages, &msg)
+	}
+
+	if err := scanner.Err(); err != nil {
+		return nil, err
+	}
+
+	// Sort by timestamp (newest first)
+	sort.Slice(messages, func(i, j int) bool {
+		return messages[i].Timestamp.After(messages[j].Timestamp)
+	})
+
+	return messages, nil
+}
+
+// ListUnread returns unread messages.
+func (m *Mailbox) ListUnread() ([]*Message, error) {
+	all, err := m.List()
+	if err != nil {
+		return nil, err
+	}
+
+	var unread []*Message
+	for _, msg := range all {
+		if !msg.Read {
+			unread = append(unread, msg)
+		}
+	}
+
+	return unread, nil
+}
+
+// Get returns a message by ID.
+func (m *Mailbox) Get(id string) (*Message, error) {
+	messages, err := m.List()
+	if err != nil {
+		return nil, err
+	}
+
+	for _, msg := range messages {
+		if msg.ID == id {
+			return msg, nil
+		}
+	}
+
+	return nil, ErrMessageNotFound
+}
+
+// Append adds a message to the mailbox.
+func (m *Mailbox) Append(msg *Message) error {
+	// Ensure directory exists
+	dir := filepath.Dir(m.path)
+	if err := os.MkdirAll(dir, 0755); err != nil {
+		return err
+	}
+
+	// Open for append
+	file, err := os.OpenFile(m.path, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+
+	data, err := json.Marshal(msg)
+	if err != nil {
+		return err
+	}
+
+	_, err = file.WriteString(string(data) + "\n")
+	return err
+}
+
+// MarkRead marks a message as read.
+func (m *Mailbox) MarkRead(id string) error {
+	messages, err := m.List()
+	if err != nil {
+		return err
+	}
+
+	found := false
+	for _, msg := range messages {
+		if msg.ID == id {
+			msg.Read = true
+			found = true
+		}
+	}
+
+	if !found {
+		return ErrMessageNotFound
+	}
+
+	return m.rewrite(messages)
+}
+
+// Delete removes a message from the mailbox.
+func (m *Mailbox) Delete(id string) error {
+	messages, err := m.List()
+	if err != nil {
+		return err
+	}
+
+	var filtered []*Message
+	found := false
+	for _, msg := range messages {
+		if msg.ID == id {
+			found = true
+		} else {
+			filtered = append(filtered, msg)
+		}
+	}
+
+	if !found {
+		return ErrMessageNotFound
+	}
+
+	return m.rewrite(filtered)
+}
+
+// Count returns the total and unread message counts.
+func (m *Mailbox) Count() (total, unread int, err error) {
+	messages, err := m.List()
+	if err != nil {
+		return 0, 0, err
+	}
+
+	total = len(messages)
+	for _, msg := range messages {
+		if !msg.Read {
+			unread++
+		}
+	}
+
+	return total, unread, nil
+}
+
+// rewrite rewrites the mailbox with the given messages.
+func (m *Mailbox) rewrite(messages []*Message) error {
+	// Sort by timestamp (oldest first for JSONL)
+	sort.Slice(messages, func(i, j int) bool {
+		return messages[i].Timestamp.Before(messages[j].Timestamp)
+	})
+
+	// Write to temp file
+	tmpPath := m.path + ".tmp"
+	file, err := os.Create(tmpPath)
+	if err != nil {
+		return err
+	}
+
+	for _, msg := range messages {
+		data, err := json.Marshal(msg)
+		if err != nil {
+			file.Close()
+			os.Remove(tmpPath)
+			return err
+		}
+		file.WriteString(string(data) + "\n")
+	}
+
+	if err := file.Close(); err != nil {
+		os.Remove(tmpPath)
+		return err
+	}
+
+	// Atomic rename
+	return os.Rename(tmpPath, m.path)
+}