docs: make the English README default with a zh-CN companion

The English README becomes README.md so the GitHub landing page reads in
English; the Chinese one moves to README.zh-CN.md. Both cross-link at the
top. Drops the source-origin framing from the intro and removes the
Disclaimer section, and points the license badge at MIT.

change-policy and pr-triage hardcoded README.en.md, which would have
dropped README.zh-CN.md out of the docs lane.

scripts/pr/change-policy.test.ts

@@ -35,6 +35,7 @@ describe('evaluateChangePolicy', () => {
       'docs/index.md',
       'site/src/App.tsx',
       'README.md',
+      'README.zh-CN.md',
     ])
 
     expect(result.blocked).toBe(false)

scripts/pr/change-policy.ts

@@ -144,7 +144,7 @@ const policyExactPaths = new Set([
 
 const docsExactPaths = new Set([
   'README.md',
-  'README.en.md',
+  'README.zh-CN.md',
   'package.json',
   'package-lock.json',
   '.github/workflows/deploy-docs.yml',