feat(downloads): fold download plans into download panel (DL-3)

- Waiting plans (approval / disk space / automation off / queued) are
  active-segment rows on /download-tasks/board, merged by created_at,
  counted, with approve / cancel actions; ?plan= deep links and legacy
  /downloads/plans routes redirect to the panel
- Download detail shows "why this download" (source, purpose, replaced
  version, reasons); raw evaluation in tech info
- Failed / cancelled / expired plans appear in the entity download
  history; rejected / revoked / expired approvals close draft plans
- New GET /acquisition-plans/{id}/why, POST /acquisition-plans/{id}/cancel
- User-facing copy no longer says 下载计划; glossary updated

Co-Authored-By: Claude Opus 5.5 <[email protected]>

apps/web/app/components/agent/agui3.test.ts

@@ -189,6 +189,8 @@ describe("聚合:进度行用能力元数据的人话计数", () => {
     expect(activitySummary([call({ capabilityId: "download.inspect" }), call({ capabilityId: "site.list" })])).toBe(
       "查了 1 个下载任务和站点",
     );
+    // DL-3:不再说"生成了 1 份下载计划"
+    expect(activitySummary([call({ capabilityId: "download.plan" })])).toBe("准备下载 1 个资源");
   });
   it("一回合的全部调用聚成一行(1 次也折叠),展开前看不到能力 ID", () => {
     const activity: ActivityItem[] = [{ kind: "tool", key: "c1", call: call({ capabilityId: "site.list" }) }];