Replace mention fan-out with backend mentions index

useMentionActivity now wraps a single getTaskMentions query against the new task_mentions endpoint; the per-channel/per-task thread scan, its mention-matching helpers, and the plan file are deleted. mentionActivity in core is reduced to a DTO-to-item mapper (countUnseenActivity unchanged), and ActivityView resolves desktop folder-channel ids itself.

Generated-By: PostHog Code
Task-Id: 47e3ad2f-36c4-4d54-ae0c-07923d306818

Author: k11kirky

docs/plans/mention-activity-backend-index.md

@@ -73,6 +73,7 @@ import type {
   SuggestedReviewerWriteEntry,
   Task,
   TaskChannel,
+  TaskMention,
   TaskRun,
   TaskRunArtefact,
   TaskThreadMessage,
@@ -2303,6 +2304,25 @@ export class PostHogAPIClient {
     return (await response.json()) as TaskChannel;
   }
 
+  // Mentions of the current user across task threads, newest first.
+  async getTaskMentions(options?: { since?: string }): Promise<TaskMention[]> {
+    const teamId = await this.getTeamId();
+    const urlPath = `/api/projects/${teamId}/task_mentions/`;
+    const url = new URL(`${this.api.baseUrl}${urlPath}`);
+    if (options?.since) {
+      url.searchParams.set("since", options.since);
+    }
+    const response = await this.api.fetcher.fetch({
+      method: "get",
+      url,
+      path: urlPath,
+    });
+    if (!response.ok) {
+      throw new Error(`Failed to fetch task mentions: ${response.statusText}`);
+    }
+    return (await response.json()) as TaskMention[];
+  }
+
   async getTaskThreadMessages(taskId: string): Promise<TaskThreadMessage[]> {
     const teamId = await this.getTeamId();
     const urlPath = `/api/projects/${teamId}/tasks/${taskId}/thread_messages/`;