feat(mail): sort pinned messages first in inbox (gt-ngu1)

Add Pinned field to Message and BeadsMessage types, and implement
sorting in listBeads() to show pinned messages first, then by
priority, then by date.

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

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

internal/mail/mailbox.go

@@ -105,9 +105,41 @@ func (m *Mailbox) listBeads() ([]*Message, error) {
 		messages = append(messages, bm.ToMessage())
 	}
 
+	// Sort: pinned first, then by priority (urgent first), then by date (newest first)
+	sort.Slice(messages, func(i, j int) bool {
+		// Pinned messages always come first
+		if messages[i].Pinned != messages[j].Pinned {
+			return messages[i].Pinned
+		}
+		// Within same pinned status, sort by priority (urgent > high > normal > low)
+		pi := priorityOrder(messages[i].Priority)
+		pj := priorityOrder(messages[j].Priority)
+		if pi != pj {
+			return pi < pj
+		}
+		// Within same priority, sort by date (newest first)
+		return messages[i].Timestamp.After(messages[j].Timestamp)
+	})
+
 	return messages, nil
 }
 
+// priorityOrder returns sort order for priority (lower = more urgent).
+func priorityOrder(p Priority) int {
+	switch p {
+	case PriorityUrgent:
+		return 0
+	case PriorityHigh:
+		return 1
+	case PriorityNormal:
+		return 2
+	case PriorityLow:
+		return 3
+	default:
+		return 2
+	}
+}
+
 func (m *Mailbox) listLegacy() ([]*Message, error) {
 	file, err := os.Open(m.path)
 	if err != nil {

internal/mail/types.go

@@ -78,6 +78,9 @@ type Message struct {
 	// Read indicates if the message has been read (closed in beads).
 	Read bool `json:"read"`
 
+	// Pinned indicates if the message is pinned (persistent context marker).
+	Pinned bool `json:"pinned,omitempty"`
+
 	// Priority is the message priority.
 	Priority Priority `json:"priority"`
 
@@ -151,6 +154,7 @@ type BeadsMessage struct {
 	Assignee    string    `json:"assignee"`    // To identity
 	Priority    int       `json:"priority"`    // 0=urgent, 1=high, 2=normal, 3=low
 	Status      string    `json:"status"`      // open=unread, closed=read
+	Pinned      bool      `json:"pinned"`      // Persistent context marker
 	CreatedAt   time.Time `json:"created_at"`
 	Type        string    `json:"type,omitempty"`      // Message type
 	ThreadID    string    `json:"thread_id,omitempty"` // Thread identifier
@@ -187,6 +191,7 @@ func (bm *BeadsMessage) ToMessage() *Message {
 		Body:      bm.Description,
 		Timestamp: bm.CreatedAt,
 		Read:      bm.Status == "closed",
+		Pinned:    bm.Pinned,
 		Priority:  priority,
 		Type:      msgType,
 		ThreadID:  bm.ThreadID,