docs: reorganize into design, security, api, ops and dev; drop stale designs; refresh the diagrams [CI]

- Group docs by category with an index (docs/README.md); fix every link,
  code comment and the protocol conformance test that read docs/protocol.md.
- access.md and security.md rewritten to what is implemented: removed the
  unadopted member-proof chain, roles and recovery key that do not exist,
  the old implementation order and the budgets; gaps now carry a status.
- enrollment, deployment, api: the install flow is the static, hash-checked
  script with environment variables; no default admin.
- README and architecture: new topology and update-flow diagrams (operator
  passkey, CI signing, registry feed, auth and trust modules).

backend/tests/MoShiro.Backend.Tests/ArchitectureTests.cs

@@ -3,7 +3,7 @@ using System.Xml.Linq;
 namespace MoShiro.Backend.Tests;
 
 /// <summary>
-/// 依赖规则（docs/project-layout.md 第 3 节）：由测试在构建期强制。
+/// 依赖规则（docs/dev/project-layout.md 第 3 节）：由测试在构建期强制。
 /// 读取仓库里所有 .csproj 和源文件，违反规则就失败。
 /// </summary>
 public sealed class ArchitectureTests