docs: fix misleading help text for gt mail read (#565)

The help text claimed 'gt mail read' marks messages as read, but this
was intentionally removed in 71d313ed to preserve handoff messages.

Update the help text to accurately reflect the current behavior and
point users to 'gt mail mark-read' for explicit read marking.

internal/cmd/mail.go

@@ -150,9 +150,10 @@ Examples:
 var mailReadCmd = &cobra.Command{
 	Use:   "read <message-id>",
 	Short: "Read a message",
-	Long: `Read a specific message and mark it as read.
+	Long: `Read a specific message (does not mark as read).
 
-The message ID can be found from 'gt mail inbox'.`,
+The message ID can be found from 'gt mail inbox'.
+Use 'gt mail mark-read' to mark messages as read.`,
 	Aliases: []string{"show"},
 	Args: cobra.ExactArgs(1),
 	RunE: runMailRead,