feat(plugin): extract rule engine and harden channel/session handling

plugin/rules.ts

@@ -0,0 +1,47 @@
+export type WhisperGateConfig = {
+  enabled?: boolean;
+  discordOnly?: boolean;
+  bypassUserIds?: string[];
+  endSymbols?: string[];
+  noReplyProvider: string;
+  noReplyModel: string;
+};
+
+export type Decision = {
+  shouldUseNoReply: boolean;
+  reason: string;
+};
+
+function getLastChar(input: string): string {
+  const t = input.trim();
+  return t.length ? t[t.length - 1] : "";
+}
+
+export function evaluateDecision(params: {
+  config: WhisperGateConfig;
+  channel?: string;
+  senderId?: string;
+  content?: string;
+}): Decision {
+  const { config } = params;
+
+  if (config.enabled === false) {
+    return { shouldUseNoReply: false, reason: "disabled" };
+  }
+
+  const channel = (params.channel || "").toLowerCase();
+  if (config.discordOnly !== false && channel !== "discord") {
+    return { shouldUseNoReply: false, reason: "non_discord" };
+  }
+
+  if (params.senderId && (config.bypassUserIds || []).includes(params.senderId)) {
+    return { shouldUseNoReply: false, reason: "bypass_sender" };
+  }
+
+  const lastChar = getLastChar(params.content || "");
+  if (lastChar && (config.endSymbols || []).includes(lastChar)) {
+    return { shouldUseNoReply: false, reason: `end_symbol:${lastChar}` };
+  }
+
+  return { shouldUseNoReply: true, reason: "rule_match_no_end_symbol" };
+}