refactor: centralize slack threading helpers

src/slack/threading.ts

@@ -0,0 +1,15 @@
+import type { ReplyToMode } from "../config/types.js";
+import type { SlackAppMentionEvent, SlackMessageEvent } from "./types.js";
+
+export function resolveSlackThreadTargets(params: {
+  message: SlackMessageEvent | SlackAppMentionEvent;
+  replyToMode: ReplyToMode;
+}) {
+  const incomingThreadTs = params.message.thread_ts;
+  const eventTs = params.message.event_ts;
+  const messageTs = params.message.ts ?? eventTs;
+  const replyThreadTs =
+    incomingThreadTs ?? (params.replyToMode === "all" ? messageTs : undefined);
+  const statusThreadTs = replyThreadTs ?? messageTs;
+  return { replyThreadTs, statusThreadTs };
+}