feat(csm): bootstrap discussion callback flow

plugin/core/discussion-state.ts

@@ -0,0 +1,55 @@
+export type DiscussionStatus = "active" | "closed";
+
+export type DiscussionMetadata = {
+  mode: "discussion";
+  discussionChannelId: string;
+  originChannelId: string;
+  initiatorAgentId: string;
+  initiatorSessionId: string;
+  discussGuide: string;
+  status: DiscussionStatus;
+  createdAt: string;
+  completedAt?: string;
+  summaryPath?: string;
+  idleReminderSent?: boolean;
+};
+
+const discussionByChannelId = new Map<string, DiscussionMetadata>();
+
+export function createDiscussion(metadata: DiscussionMetadata): DiscussionMetadata {
+  discussionByChannelId.set(metadata.discussionChannelId, metadata);
+  return metadata;
+}
+
+export function getDiscussion(channelId: string): DiscussionMetadata | undefined {
+  return discussionByChannelId.get(channelId);
+}
+
+export function isDiscussionChannel(channelId: string): boolean {
+  return discussionByChannelId.has(channelId);
+}
+
+export function isDiscussionClosed(channelId: string): boolean {
+  return discussionByChannelId.get(channelId)?.status === "closed";
+}
+
+export function markDiscussionIdleReminderSent(channelId: string): void {
+  const rec = discussionByChannelId.get(channelId);
+  if (!rec) return;
+  rec.idleReminderSent = true;
+}
+
+export function clearDiscussionIdleReminderSent(channelId: string): void {
+  const rec = discussionByChannelId.get(channelId);
+  if (!rec) return;
+  rec.idleReminderSent = false;
+}
+
+export function closeDiscussion(channelId: string, summaryPath: string): DiscussionMetadata | undefined {
+  const rec = discussionByChannelId.get(channelId);
+  if (!rec) return undefined;
+  rec.status = "closed";
+  rec.summaryPath = summaryPath;
+  rec.completedAt = new Date().toISOString();
+  return rec;
+}