docs: audit the docs, merge the Fleet docs into backend/fleet/README.md and add TODO.md [CI SKIP]

Drop the unimplemented fleet updater design, the stale project-layout sections and broken references, fix the API table, and rename the four Monitor*.cs files left over from the rename.

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，等价于文档里的 <c>eng/check-deps</c>。
+/// 依赖规则（docs/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