docs: 删掉 README「nothing to install」假声明，改正 peer 叙述

d144284 补了「Prerequisite: link the harness packages first」章节，但留下两处
与新章节自相矛盾的话，这一提交收掉：

1. README §Install 的安装注释 `npm install  # nothing to install; dsh packages
   are peers` —— 直接删。实测它不成立：干净副本里 npm install 只拉到 cordis
   （四个 peer 里唯一非 optional 的）加它的传递依赖，dsh-llm /
   dsh-system-prompt / dsh-tools 三个仍然缺；不 install 更彻底，四个全缺。
   改成说明 peer 归属 + 指向前置章节的链接。
2. 新章节开头 `This repository has **no runtime dependencies**` —— 同样不成立
   （lib/index.js 运行时 import 三个 @deepseek-ai 包）。改成「不以本插件自身
   依赖的形式拉取任何东西」，把 peer 归属说清，并写明 npm install 只解决
   cordis 一个。
3. 症状段补一句 npm run check 同样红（实测 exit=1，checkpoints.js 在 import
   阶段就 ERR_MODULE_NOT_FOUND），原文只提了 npm test。
4. scripts/link-dsh.sh 头部注释里的 `zero runtime dependencies` 一并改成同义
   但不撒谎的说法。

验证（自己跑，不转述）
- 干净副本 git archive HEAD + 本补丁，不 link：
    not ok loader / phase-b-g / phase-b-h / plugin
    # tests 34  # pass 30  # fail 4
    npm run check exit=1（Cannot find package '@deepseek-ai/cordis'）
- 同一副本 bash scripts/link-dsh.sh 之后：
    # tests 120  # pass 120  # fail 0
    npm run check exit=0，10 条 ✔
- npm install 单独一步（不 link）也复现：四文件 ERR_MODULE_NOT_FOUND，
  缺的是 dsh-system-prompt / dsh-llm（cordis 已装）
- README 全文再无「nothing to install / no runtime dependencies / zero
  runtime dependencies」类断言（grep 零命中）
- README 计数禁令仍绿，无 .orig/.rej/.bak 入库

更正一处旧结论：交接说「不 link 时 4 个文件 ERR_MODULE_NOT_FOUND」= 少 4 个
用例，不对。那四个文件在 import 阶段就死，用例根本没注册，所以 npm test 汇总
是 34 tests，不是 116/120。已在 README 里写成「a handful of tests rather than
the full suite」，避免再误导。

README.md

@@ -177,9 +177,16 @@ something", and no waterfall that can rewrite a loop-built request
 
 ## Install
 
+The plugin brings no dependency of its own: everything it imports at runtime is a
+`@deepseek-ai/*` **peer** that ships with the dsh install on the machine.
+`npm install` does **not** deliver those peers (three of the four are declared
+optional), so on its own it leaves a tree that cannot even load the plugin — link
+them first, per
+[Prerequisite: link the harness packages first](#prerequisite-link-the-harness-packages-first).
+
 ```bash
 # from this repo
-npm install                    # nothing to install; dsh packages are peers
+npm install
 ```
 
 The plugin needs `node:sqlite` (built into Node ≥ 22.5). On Node 22.23 it works
@@ -646,22 +653,26 @@ because they will bite anyone writing a second dsh service plugin.
 
 ### Prerequisite: link the harness packages first
 
-This repository has **no runtime dependencies** — `lib/` uses only `node:`
-built-ins. Everything it imports from `@deepseek-ai/*` is a **peer** that must
-come from the dsh install already on the machine, which is why none of it is
-bundled here (vendoring a copy would let it drift from the harness it is supposed
-to run inside).
+Nothing here is fetched as this plugin's own dependency. The `@deepseek-ai/*`
+packages that `lib/` imports at runtime are **peers**: they belong to the dsh
+install already on the machine, and they are not vendored here on purpose
+(a copy would drift from the harness this plugin is supposed to run inside).
+`npm install` does not resolve them either — `cordis` is the only one of the four
+that is non-optional, so npm fetches that one (plus its own transitive deps) and
+leaves the other three missing.
 
-So a fresh clone needs one setup step before the suite can run:
+A fresh clone therefore needs one setup step before the suite can run:
 
 ```bash
 bash scripts/link-dsh.sh          # symlinks the installed dsh packages into node_modules
 ```
 
-**If you skip it, `npm test` fails with four files reporting
-`ERR_MODULE_NOT_FOUND: Cannot find package '@deepseek-ai/cordis'`** (loader,
-plugin, guard and http suites). That is the expected symptom of a missing setup
-step, not a broken tree — run the script and it goes green.
+**Skip it and `npm test` and `npm run check` both fail with four files
+reporting `ERR_MODULE_NOT_FOUND: Cannot find package '@deepseek-ai/…'`** —
+`loader`, `plugin`, `phase-b-g` and `phase-b-h` (the suites that mount a real
+dsh app). Those four die at import, so the run reports a handful of tests rather
+than the full suite. That is the expected symptom of a missing setup step, not a
+broken tree; run the script and both commands go green.
 
 The script takes an optional argument for a non-default install root:
 

scripts/link-dsh.sh

@@ -1,9 +1,10 @@
 #!/usr/bin/env bash
 # Link the globally-installed dsh packages into node_modules.
 #
-# The plugin itself has zero runtime dependencies (it uses only `node:` built-ins),
-# so nothing is installed from a registry. The dsh packages are *peers*: they must
-# come from the running harness. During development they live in
+# The plugin declares no dependency of its own, so nothing here is fetched from a
+# registry. The dsh packages it *imports* at runtime are **peers**: they must come
+# from the running harness, and npm will not install them for us. During
+# development they live in
 # /usr/lib/node_modules/@deepseek-ai (a global install), so this script wires them
 # up by symlink instead of vendoring a copy that could drift from the harness.
 #