docs(server-mode): plan moved in from the deleted AgentHub-Server repo; P6 spike result; Express patch only on 4.x

- docs/plans/server-mode/README.md: the original multi-user design, now kept
  here (AgentHub is the one source); desktop-parity and S3-PLAN link to it.
- S3-PLAN P6: managed-settings spike run on agh-test1 — the managed
  marketplace/plugin arrive at the first interactive session (onboarding
  seeded), never via the CLI alone, and no user-scope record is written.
- server/index.js: the Layer.handle_request patch applies only on Express 4
  (Express 5 handles async rejections itself and has no lib/router/layer).

docs/plans/desktop-parity/README.md

@@ -2,8 +2,8 @@
 
 _Status: written 2026-10-02. Phases 1–5 and 7 implemented and committed
 (a6bd33b); phase 6 implemented 2026-10-02 the Claude way (D4). Phases run in order, each one change-set on `main`.
-Comes BEFORE the multi-user server plan (now in the AgentHub-Server repo,
-https://git.smartgift.io.vn/Smartgift-AI/AgentHub-Server) — that plan's owner column hangs
+Comes BEFORE the multi-user server plan ([`docs/plans/server-mode/README.md`](../server-mode/README.md))
+— that plan's owner column hangs
 off the session ownership built in phase 1 here._
 
 ## Where we stand