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

README.md

@@ -28,8 +28,6 @@ MoShiro 旨在解决跨网络线路质量难以量化的问题：
                         测量会话：Probe → Agent
 ```
 
-Probe 与 Agent 只做测量与上报，没有终端、命令执行和自更新，更新由外部服务管理器完成。节点与 Backend 之间的身份、任务和更新均基于签名校验，详见[安全文档](docs/security/security.md)。
-
 ## 快速开始
 
 环境：.NET SDK 10（版本见 `global.json`）、Node.js 与 npm。
@@ -53,7 +51,7 @@ npm run build --prefix web
 
 | 主题 | 文档 |
 | --- | --- |
-| 架构与设计决策 | [architecture](docs/design/architecture.md) |
+| 架构与数据链路 | [architecture](docs/design/architecture.md) |
 | 线协议与方法 | [protocol](docs/design/protocol.md) |
 | 采集引擎与任务 | [measurement](docs/design/measurement.md) |
 | 分析、存储、查询 | [analysis](docs/design/analysis.md) · [storage](docs/design/storage.md) · [query](docs/design/query.md) |