emlbuilder: add LMSReplyType method for reply/forward type header

Add LMSReplyType method to the EML builder that sets the
X-LMS-Reply-Type header. This allows reply, reply-all, and forward
shortcuts to tag the EML with REPLY or FORWARD so that downstream
services can label the original message accordingly.

Call LMSReplyType("REPLY") in mail_reply.go and mail_reply_all.go,
and LMSReplyType("FORWARD") in mail_forward.go, immediately after
the existing LMSReplyToMessageID call.

shortcuts/mail/emlbuilder/builder.go

@@ -57,6 +57,10 @@ import (
 	"github.com/larksuite/cli/shortcuts/mail/filecheck"
 )
 
+// lmsReplyTypeHeaderKey is the MIME header name for the reply type
+// ("REPLY" or "FORWARD"), read by data-access to populate BodyExtra.ReplyType.
+const lmsReplyTypeHeaderKey = "X-LMS-Reply-Type"
+
 // MaxEMLSize is the maximum allowed raw EML size in bytes.
 const MaxEMLSize = 25 * 1024 * 1024 // 25 MB
 
@@ -90,6 +94,7 @@ type Builder struct {
 	inReplyTo                 string // raw value, without angle brackets
 	references                string // space-separated list of message IDs, with angle brackets
 	lmsReplyToMessageID       string // Lark internal message_id of the original message
+	lmsReplyType              string // "REPLY" or "FORWARD", written as X-LMS-Reply-Type
 	textBody                  []byte
 	htmlBody                  []byte
 	calendarBody              []byte
@@ -395,6 +400,22 @@ func (b Builder) LMSReplyToMessageID(id string) Builder {
 	return b
 }
 
+// LMSReplyType sets the reply type header X-LMS-Reply-Type.
+// t must be "REPLY" or "FORWARD"; other values are silently ignored.
+// This header is written unconditionally when set, independent of In-Reply-To.
+// Returns an error builder if t contains CR, LF, or other dangerous characters.
+func (b Builder) LMSReplyType(t string) Builder {
+	if b.err != nil {
+		return b
+	}
+	if err := validateHeaderValue(t); err != nil {
+		b.err = err
+		return b
+	}
+	b.lmsReplyType = t
+	return b
+}
+
 // References sets the References header value verbatim.
 // Typically a space-separated list of message IDs including angle brackets,
 // e.g. "<id1@host> <id2@host>".
@@ -732,6 +753,9 @@ func (b Builder) Build() ([]byte, error) {
 	for _, kv := range b.extraHeaders {
 		writeHeader(&buf, kv[0], kv[1])
 	}
+	if b.lmsReplyType != "" {
+		writeHeader(&buf, lmsReplyTypeHeaderKey, b.lmsReplyType)
+	}
 
 	// ── Body ───────────────────────────────────────────────────────────────────
 	// Full MIME hierarchy (outer layers only present when needed):

shortcuts/mail/emlbuilder/builder_test.go

@@ -285,6 +285,48 @@ func TestBuild_LMSReplyToMessageID_NotWrittenWithoutInReplyTo(t *testing.T) {
 	}
 }
 
+// TestBuild_LMSReplyType verifies LMS reply type header writing.
+func TestBuild_LMSReplyType(t *testing.T) {
+	raw, err := New().
+		From("", "alice@example.com").
+		To("", "bob@example.com").
+		Subject("Re: hello").
+		Date(fixedDate).
+		LMSReplyType("REPLY").
+		TextBody([]byte("my reply")).
+		Build()
+	if err != nil {
+		t.Fatal(err)
+	}
+	eml := string(raw)
+
+	got := headerValue(eml, "X-LMS-Reply-Type")
+	if got != "REPLY" {
+		t.Errorf("X-LMS-Reply-Type: got %q, want REPLY", got)
+	}
+}
+
+// TestBuild_LMSReplyType_Forward verifies LMSReplyType with FORWARD value.
+func TestBuild_LMSReplyType_Forward(t *testing.T) {
+	raw, err := New().
+		From("", "alice@example.com").
+		To("", "bob@example.com").
+		Subject("Fw: hello").
+		Date(fixedDate).
+		LMSReplyType("FORWARD").
+		TextBody([]byte("fw body")).
+		Build()
+	if err != nil {
+		t.Fatal(err)
+	}
+	eml := string(raw)
+
+	got := headerValue(eml, "X-LMS-Reply-Type")
+	if got != "FORWARD" {
+		t.Errorf("X-LMS-Reply-Type: got %q, want FORWARD", got)
+	}
+}
+
 // ── Disposition-Notification-To (read receipt) ───────────────────────────────
 
 // TestBuild_DispositionNotificationTo verifies build disposition notification to.

shortcuts/mail/mail_forward.go

@@ -256,6 +256,7 @@ var MailForward = common.Shortcut{
 		}
 		if messageId != "" {
 			bld = bld.LMSReplyToMessageID(messageId)
+			bld = bld.LMSReplyType("FORWARD")
 		}
 		useHTML := !plainText && (bodyIsHTML(body) || bodyIsHTML(orig.bodyRaw) || sigResult != nil)
 		if strings.TrimSpace(inlineFlag) != "" && !useHTML {

shortcuts/mail/mail_reply.go

@@ -269,6 +269,7 @@ var MailReply = common.Shortcut{
 		}
 		if messageId != "" {
 			bld = bld.LMSReplyToMessageID(messageId)
+			bld = bld.LMSReplyType("REPLY")
 		}
 		var autoResolvedPaths []string
 		var composedHTMLBody string

shortcuts/mail/mail_reply_all.go

@@ -278,6 +278,7 @@ var MailReplyAll = common.Shortcut{
 		}
 		if messageId != "" {
 			bld = bld.LMSReplyToMessageID(messageId)
+			bld = bld.LMSReplyType("REPLY")
 		}
 		var autoResolvedPaths []string
 		var composedHTMLBody string