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/fleet/src/Fleet.Identity/OperatorTrust.cs

@@ -20,7 +20,7 @@ public sealed record OperatorKey(string CredentialId, byte[] PublicKeySpki);
 public sealed record OperatorSet(int Version, string Previous, string RpId, IReadOnlyList<string> Origins, IReadOnlyList<OperatorKey> Keys, IReadOnlyList<byte[]>? Servers = null);
 
 /// <summary>
-/// 信任根：操作员（passkey）→ 操作员集合 → 节点本地固定的属主。设计见 docs/access.md 第 7 节。
+/// 信任根：操作员（passkey）→ 操作员集合 → 节点本地固定的属主。设计见 docs/security/access.md 第 8 节。
 /// 安装命令里带着创世指纹和属主（passkey 的凭证 id）：节点把它们固定在本地，之后只认从创世出发、由操作员 passkey 签过名的集合版本，
 /// 属主必须是集合里的凭证。Backend 能转发集合，但签不出新的版本，也改不了节点已经固定的东西。
 /// 全是纯函数，只用 BCL，节点和 Backend 共用。语句是 <c>key=value</c> 的文本，第一行是带版本的标题；签名覆盖整段文本。