refactor(plugin): extract identity and mention helpers into core modules

plugin/core/mentions.ts

@@ -0,0 +1,31 @@
+import type { DirigentConfig } from "../rules.js";
+
+function userIdFromToken(token: string): string | undefined {
+  try {
+    const segment = token.split(".")[0];
+    const padded = segment + "=".repeat((4 - (segment.length % 4)) % 4);
+    return Buffer.from(padded, "base64").toString("utf8");
+  } catch {
+    return undefined;
+  }
+}
+
+export function extractMentionedUserIds(content: string): string[] {
+  const regex = /<@!?(\d+)>/g;
+  const ids: string[] = [];
+  const seen = new Set<string>();
+  let match: RegExpExecArray | null;
+  while ((match = regex.exec(content)) !== null) {
+    const id = match[1];
+    if (!seen.has(id)) {
+      seen.add(id);
+      ids.push(id);
+    }
+  }
+  return ids;
+}
+
+export function getModeratorUserId(config: DirigentConfig): string | undefined {
+  if (!config.moderatorBotToken) return undefined;
+  return userIdFromToken(config.moderatorBotToken);
+}