docs: add the implementation plan for issue #7

Five tasks, with the ARCHITECTURE section 6.1 revision first so the code
never contradicts the single source of truth, and the reparenting kept
atomic because scope is a required keyword argument and any split would
leave an unrunnable tree.

The Codex review caught that the planned test evidence pointed at the
wrong stubs: the ones at test_backpressure.py:176-186 cover accounting-side
degradation, not the three gate paths that actually leak to callers, and
try_acquire and try_enter have no stub at all.

research-wiki/graph/edges.json

@@ -135,6 +135,11 @@
       "id": "design:governance-backend-error",
       "label": "治理后端故障归位为 scope 级不可用(Issue #7)",
       "type": "design"
+    },
+    {
+      "id": "plan:governance-backend-error",
+      "label": "实现计划: 治理后端故障归位为 scope 级不可用(Issue #7)",
+      "type": "plan"
     }
   ],
   "links": [
@@ -242,6 +247,13 @@
       "relation": "implements",
       "evidence": "T1-T10 逐条实现设计的 D1-D6 六个决策与 §11 九条验收标准",
       "added": "2026-08-02T09:49:48.126539+00:00"
+    },
+    {
+      "source": "plan:governance-backend-error",
+      "target": "design:governance-backend-error",
+      "relation": "implements",
+      "evidence": "T1-T5 逐任务实现设计 §3 的五项决策与 §8 影响面清单",
+      "added": "2026-08-06T08:08:51.865565+00:00"
     }
   ]
 }