feat: add gt mail mark-read command for desire path (bd-rjuu6)

Adds mark-read and mark-unread commands that allow marking messages
as read without archiving them. Uses a "read" label to track status.

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

internal/mail/types.go

@@ -256,7 +256,7 @@ func (bm *BeadsMessage) ToMessage() *Message {
 		Subject:   bm.Title,
 		Body:      bm.Description,
 		Timestamp: bm.CreatedAt,
-		Read:      bm.Status == "closed",
+		Read:      bm.Status == "closed" || bm.HasLabel("read"),
 		Priority:  priority,
 		Type:      msgType,
 		ThreadID:  bm.threadID,
@@ -266,6 +266,16 @@ func (bm *BeadsMessage) ToMessage() *Message {
 	}
 }
 
+// HasLabel checks if the message has a specific label.
+func (bm *BeadsMessage) HasLabel(label string) bool {
+	for _, l := range bm.Labels {
+		if l == label {
+			return true
+		}
+	}
+	return false
+}
+
 // PriorityToBeads converts a GGT Priority to beads priority integer.
 // Returns: 0=urgent, 1=high, 2=normal, 3=low
 func PriorityToBeads(p Priority) int {