fix(discord): support applied_tags parameter for forum thread creation

Forum channels that require tags fail with "A tag is required" when
creating threads because there was no way to pass tag IDs. Add
appliedTags parameter to the thread-create action so forum posts can
include required tags from the channel's available_tags list.

src/discord/send.creates-thread.test.ts

@@ -76,6 +76,44 @@ describe("sendMessageDiscord", () => {
     );
   });
 
+  it("passes applied_tags for forum threads", async () => {
+    const { rest, getMock, postMock } = makeDiscordRest();
+    getMock.mockResolvedValue({ type: ChannelType.GuildForum });
+    postMock.mockResolvedValue({ id: "t1" });
+    await createThreadDiscord(
+      "chan1",
+      { name: "tagged post", appliedTags: ["tag1", "tag2"] },
+      { rest, token: "t" },
+    );
+    expect(postMock).toHaveBeenCalledWith(
+      Routes.threads("chan1"),
+      expect.objectContaining({
+        body: {
+          name: "tagged post",
+          message: { content: "tagged post" },
+          applied_tags: ["tag1", "tag2"],
+        },
+      }),
+    );
+  });
+
+  it("omits applied_tags for non-forum threads", async () => {
+    const { rest, getMock, postMock } = makeDiscordRest();
+    getMock.mockResolvedValue({ type: ChannelType.GuildText });
+    postMock.mockResolvedValue({ id: "t1" });
+    await createThreadDiscord(
+      "chan1",
+      { name: "thread", appliedTags: ["tag1"] },
+      { rest, token: "t" },
+    );
+    expect(postMock).toHaveBeenCalledWith(
+      Routes.threads("chan1"),
+      expect.objectContaining({
+        body: expect.not.objectContaining({ applied_tags: expect.anything() }),
+      }),
+    );
+  });
+
   it("falls back when channel lookup is unavailable", async () => {
     const { rest, getMock, postMock } = makeDiscordRest();
     getMock.mockRejectedValue(new Error("lookup failed"));