docs: tighten wording, soften node capability claims, link architecture to code

- Remove the node capability paragraph from README; soften "pure collector"
  and "no self-update" statements in docs, AGENTS.md and READMEs
- architecture.md: drop scalability goals, design decisions and open items;
  add a component-to-code table and code links
- Replace rhetorical contrasts in security docs with direct statements
- AGENTS.md: add project mental model, change map and writing style rules

docs/dev/project-layout.md

@@ -25,7 +25,7 @@ moshiro/
 └─ backend/                                   # 服务和节点 C# 代码（构建产物在 backend/artifacts/）
    ├─ Directory.Build.props  MoShiro.slnx      # 后端编译设置和后端目录解决方案
    ├─ .editorconfig  BannedSymbols.txt         # 风格与禁用 API（构建期强制）
-   ├─ BannedSymbols.Node.txt                   # 仅用于常驻节点进程：纯采集边界
+   ├─ BannedSymbols.Node.txt                   # 仅用于常驻节点进程：限制节点能力面
    ├─ fleet/                                  # 通用节点框架（不认识 MoShiro，以后可以拆成独立仓库）
    │  ├─ src/     Fleet.Core Fleet.Identity Fleet.Rpc Fleet.Link Fleet.Hosting
    │  │           Fleet.Enrollment.Client  Fleet.Server.{Abstractions,Enrollment,Link}