docs: add the missing imports to the plan's key interfaces

Codex review: the code blocks reference httpx and PolyGatewayError, but
neither module imports them today. A zero-context implementer copying
them verbatim would stall on F821.

research-wiki/graph/edges.json

@@ -155,6 +155,11 @@
       "id": "design:issue10-error-body-retention",
       "label": "HTTP 错误响应体留存(Issue #10)",
       "type": "design"
+    },
+    {
+      "id": "plan:issue10-error-body-retention-plan",
+      "label": "实现计划: HTTP 错误响应体留存(Issue #10)",
+      "type": "plan"
     }
   ],
   "links": [
@@ -276,6 +281,13 @@
       "relation": "implements",
       "evidence": "T1-T6 实施该设计,含 §3.6 订正",
       "added": "2026-08-06T14:58:01.673693+00:00"
+    },
+    {
+      "source": "plan:issue10-error-body-retention-plan",
+      "target": "design:issue10-error-body-retention",
+      "relation": "implements",
+      "evidence": "7 任务覆盖设计 G1-G4 与 §7 全部验收用例",
+      "added": "2026-08-16T09:50:57.830855+00:00"
     }
   ]
 }