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/cmd/mail.go

@@ -191,6 +191,38 @@ Examples:
 	RunE: runMailArchive,
 }
 
+var mailMarkReadCmd = &cobra.Command{
+	Use:   "mark-read <message-id> [message-id...]",
+	Short: "Mark messages as read without archiving",
+	Long: `Mark one or more messages as read without removing them from inbox.
+
+This adds a 'read' label to the message, which is reflected in the inbox display.
+The message remains in your inbox (unlike archive which closes/removes it).
+
+Use case: You've read a message but want to keep it visible in your inbox
+for reference or follow-up.
+
+Examples:
+  gt mail mark-read hq-abc123
+  gt mail mark-read hq-abc123 hq-def456`,
+	Args: cobra.MinimumNArgs(1),
+	RunE: runMailMarkRead,
+}
+
+var mailMarkUnreadCmd = &cobra.Command{
+	Use:   "mark-unread <message-id> [message-id...]",
+	Short: "Mark messages as unread",
+	Long: `Mark one or more messages as unread.
+
+This removes the 'read' label from the message.
+
+Examples:
+  gt mail mark-unread hq-abc123
+  gt mail mark-unread hq-abc123 hq-def456`,
+	Args: cobra.MinimumNArgs(1),
+	RunE: runMailMarkUnread,
+}
+
 var mailCheckCmd = &cobra.Command{
 	Use:   "check",
 	Short: "Check for new mail (for hooks)",
@@ -438,6 +470,8 @@ func init() {
 	mailCmd.AddCommand(mailPeekCmd)
 	mailCmd.AddCommand(mailDeleteCmd)
 	mailCmd.AddCommand(mailArchiveCmd)
+	mailCmd.AddCommand(mailMarkReadCmd)
+	mailCmd.AddCommand(mailMarkUnreadCmd)
 	mailCmd.AddCommand(mailCheckCmd)
 	mailCmd.AddCommand(mailThreadCmd)
 	mailCmd.AddCommand(mailReplyCmd)

internal/cmd/mail_inbox.go

@@ -292,6 +292,98 @@ func runMailArchive(cmd *cobra.Command, args []string) error {
 	return nil
 }
 
+func runMailMarkRead(cmd *cobra.Command, args []string) error {
+	// Determine which inbox
+	address := detectSender()
+
+	// All mail uses town beads (two-level architecture)
+	workDir, err := findMailWorkDir()
+	if err != nil {
+		return fmt.Errorf("not in a Gas Town workspace: %w", err)
+	}
+
+	// Get mailbox
+	router := mail.NewRouter(workDir)
+	mailbox, err := router.GetMailbox(address)
+	if err != nil {
+		return fmt.Errorf("getting mailbox: %w", err)
+	}
+
+	// Mark all specified messages as read
+	marked := 0
+	var errors []string
+	for _, msgID := range args {
+		if err := mailbox.MarkReadOnly(msgID); err != nil {
+			errors = append(errors, fmt.Sprintf("%s: %v", msgID, err))
+		} else {
+			marked++
+		}
+	}
+
+	// Report results
+	if len(errors) > 0 {
+		fmt.Printf("%s Marked %d/%d messages as read\n",
+			style.Bold.Render("⚠"), marked, len(args))
+		for _, e := range errors {
+			fmt.Printf("  Error: %s\n", e)
+		}
+		return fmt.Errorf("failed to mark %d messages", len(errors))
+	}
+
+	if len(args) == 1 {
+		fmt.Printf("%s Message marked as read\n", style.Bold.Render("✓"))
+	} else {
+		fmt.Printf("%s Marked %d messages as read\n", style.Bold.Render("✓"), marked)
+	}
+	return nil
+}
+
+func runMailMarkUnread(cmd *cobra.Command, args []string) error {
+	// Determine which inbox
+	address := detectSender()
+
+	// All mail uses town beads (two-level architecture)
+	workDir, err := findMailWorkDir()
+	if err != nil {
+		return fmt.Errorf("not in a Gas Town workspace: %w", err)
+	}
+
+	// Get mailbox
+	router := mail.NewRouter(workDir)
+	mailbox, err := router.GetMailbox(address)
+	if err != nil {
+		return fmt.Errorf("getting mailbox: %w", err)
+	}
+
+	// Mark all specified messages as unread
+	marked := 0
+	var errors []string
+	for _, msgID := range args {
+		if err := mailbox.MarkUnreadOnly(msgID); err != nil {
+			errors = append(errors, fmt.Sprintf("%s: %v", msgID, err))
+		} else {
+			marked++
+		}
+	}
+
+	// Report results
+	if len(errors) > 0 {
+		fmt.Printf("%s Marked %d/%d messages as unread\n",
+			style.Bold.Render("⚠"), marked, len(args))
+		for _, e := range errors {
+			fmt.Printf("  Error: %s\n", e)
+		}
+		return fmt.Errorf("failed to mark %d messages", len(errors))
+	}
+
+	if len(args) == 1 {
+		fmt.Printf("%s Message marked as unread\n", style.Bold.Render("✓"))
+	} else {
+		fmt.Printf("%s Marked %d messages as unread\n", style.Bold.Render("✓"), marked)
+	}
+	return nil
+}
+
 func runMailClear(cmd *cobra.Command, args []string) error {
 	// Determine which inbox to clear (target arg or auto-detect)
 	address := ""