docs: fold the Codex review into the issue #7 design and register it

Pins the ARCHITECTURE section 6.1 revision to land before or with the
implementation, since the new scope reason contradicts the current single
source of truth. Documents why SourceNotConfiguredError may sit outside
the four-way classification: that rule governs transport-translated call
failures, and the GatewayUnavailableError family already lives outside it.

Also collapses the ten per-field response ternaries in emit_attempt into
an _AttemptUsage view. They all expressed the same decision and pushed the
method to cyclomatic complexity C, which blocked the commit gate.

research-wiki/index.md

@@ -1,8 +1,8 @@
 # Research Wiki 索引
 
-> 自动生成，更新时间：2026-08-02 10:55 UTC
+> 自动生成，更新时间：2026-08-06 06:38 UTC
 
-## design (21)
+## design (23)
 - [2026-07-20-m1-core-design](designs/2026-07-20-m1-core-design.md) `design:2026-07-20-m1-core-design`
 - [2026-07-20-m2-distributed-design](designs/2026-07-20-m2-distributed-design.md) `design:2026-07-20-m2-distributed-design`
 - [2026-07-21-m25-resilience-design](designs/2026-07-21-m25-resilience-design.md) `design:2026-07-21-m25-resilience-design`
@@ -13,6 +13,7 @@
 - [2026-07-30-settings-invariants-round-2-design](designs/2026-07-30-settings-invariants-round-2-design.md) `design:2026-07-30-settings-invariants-round-2-design`
 - [2026-07-31-response-observability-fields-design](designs/2026-07-31-response-observability-fields-design.md) `design:2026-07-31-response-observability-fields-design`
 - [2026-07-31-sampling-params-design](designs/2026-07-31-sampling-params-design.md) `design:2026-07-31-sampling-params-design`
+- [2026-08-06-governance-backend-error-design](designs/2026-08-06-governance-backend-error-design.md) `design:2026-08-06-governance-backend-error-design`
 - [est_tokens 解耦: 拆分限流预扣与遥测用量兜底(issue #2)](designs/est-tokens-decoupling.md) `design:est-tokens-decoupling`
 - [GatewaySettings 装配校验补齐(第二轮)](designs/settings-invariants-round-2.md) `design:settings-invariants-round-2`
 - [GatewaySettings 跨字段不变量守卫的生效范围](designs/settings-invariant-guards.md) `design:settings-invariant-guards`
@@ -23,6 +24,7 @@
 - [M4 迁移验证设计(GovDoc→CHS,发 v1.0)](designs/m4-migration.md) `design:m4-migration`
 - [响应可观测字段扩展(Issue #3)](designs/response-observability-fields.md) `design:response-observability-fields`
 - [推理开关能力建模与 reasoning_tokens 采集(issue #5 + #6)](designs/2026-08-02-thinking-capability-design.md) `design:2026-08-02-thinking-capability-design`
+- [治理后端故障归位为 scope 级不可用(Issue #7)](designs/governance-backend-error.md) `design:governance-backend-error`
 - [采样参数透传设计(issue #4)](designs/sampling-params.md) `design:sampling-params`
 
 ## finding (12)