docs: add hands-on Cordis tutorial

Seven-chapter tutorial under docs/cordis-tutorial/ for agent developers
new to Cordis: first plugin, lifecycle/effects, services, events,
config, composition/HMR, and a final chapter registering a tool against
real harness services. Every transcript was produced by running the
chapter files in a gitignored tmp/ scratch directory.

Published to both website locales as mirrored English pages under a new
'Cordis tutorial' develop-sidebar section; a Chinese pair can be added
later without route changes.

website/.vitepress/config.ts

@@ -13,6 +13,7 @@ const sectionOrder = [
   '基础',
   '框架能力',
   '实战',
+  'Cordis 教程',
   '概念',
   '生成参考',
   'Cordis API',
@@ -22,6 +23,7 @@ const sectionOrder = [
   'Basics',
   'Framework',
   'Practice',
+  'Cordis tutorial',
   'Concepts',
   'Generated reference',
   'Cordis Core API',