docs(readme): make Chinese the default README and keep only the film (#1436)

- README.md is now Chinese; the English README moves to README.en.md.
- Drop the eight-screenshot grid and the Agent Teams loop; the film cover
  remains the single visual. Remove the now-unused thumbnails.
- Point PR triage, change policy, and the docs check at the new file names.

scripts/pr/change-policy.test.ts

@@ -35,7 +35,7 @@ describe('evaluateChangePolicy', () => {
       'docs/index.md',
       'site/src/App.tsx',
       'README.md',
-      'README.zh-CN.md',
+      'README.en.md',
     ])
 
     expect(result.blocked).toBe(false)

scripts/pr/change-policy.ts

@@ -177,7 +177,7 @@ const policyExactPaths = new Set([
 
 const docsExactPaths = new Set([
   'README.md',
-  'README.zh-CN.md',
+  'README.en.md',
   'package.json',
   'package-lock.json',
   '.github/workflows/deploy-docs.yml',