fix(mattermost): honor replyToMode off for threaded messages

extensions/mattermost/src/mattermost/monitor.test.ts

@@ -169,6 +169,17 @@ describe("resolveMattermostEffectiveReplyToId", () => {
     ).toBe("thread-root-456");
   });
 
+  it("suppresses existing thread roots when replyToMode is off", () => {
+    expect(
+      resolveMattermostEffectiveReplyToId({
+        kind: "channel",
+        postId: "post-123",
+        replyToMode: "off",
+        threadRootId: "thread-root-456",
+      }),
+    ).toBeUndefined();
+  });
+
   it("starts a thread for top-level channel messages when replyToMode is all", () => {
     expect(
       resolveMattermostEffectiveReplyToId({
@@ -232,6 +243,22 @@ describe("resolveMattermostThreadSessionContext", () => {
     });
   });
 
+  it("keeps threaded messages top-level when replyToMode is off", () => {
+    expect(
+      resolveMattermostThreadSessionContext({
+        baseSessionKey: "agent:main:mattermost:default:chan-1",
+        kind: "group",
+        postId: "post-123",
+        replyToMode: "off",
+        threadRootId: "root-456",
+      }),
+    ).toEqual({
+      effectiveReplyToId: undefined,
+      sessionKey: "agent:main:mattermost:default:chan-1",
+      parentSessionKey: undefined,
+    });
+  });
+
   it("keeps direct-message sessions linear", () => {
     expect(
       resolveMattermostThreadSessionContext({

extensions/mattermost/src/mattermost/monitor.ts

@@ -175,7 +175,7 @@ export function resolveMattermostEffectiveReplyToId(params: {
   threadRootId?: string | null;
 }): string | undefined {
   const threadRootId = params.threadRootId?.trim();
-  if (threadRootId) {
+  if (threadRootId && params.replyToMode !== "off") {
     return threadRootId;
   }
   if (params.kind === "direct") {