docs: update all docs with dev server, live reload, Juice inlining

- README.md: restructure Preview section into Static + Dev modes
  with feature comparison table; mention juice inlining and live reload
- CONTRIBUTING.md: update Development Setup with Go/Node.js
  requirements, static vs dev preview workflows, Juice reference
- AGENTS.md: add dev command to subcommand list, document
  tools/server/ directory, update Build Tool section
- docs/CONTRIBUTING.*.md (5 languages): add dev server section,
  Node.js requirement, live reload description
- docs/README.*.md (5 languages): add dev mode to Preview section
- docs/images/README.md: update capture instructions for dev server

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/README.zh-CN.md

@@ -50,11 +50,18 @@ systemctl restart gitea
 
 ## 预览
 
+**静态模式：**
 ```bash
 cd tools && go run . preview all
 ```
 然后打开 `preview/index.html`。
 
+**开发服务器（实时重载 + Juice CSS 内联）：**
+```bash
+cd tools && go run . dev     # 需要 Node.js
+# → http://localhost:3456
+```
+
 ---
 
 ## 兼容性