feat(repo): add CLA mechanism for external contributors

Introduce a Contributor License Agreement (CLA) so external contributions
can be licensed under AGPL-3.0 and any other terms (incl. closed-source),
keeping the AGPL-3.0 codebase usable in closed-source projects.

- CLA.md: authoritative English CLA (ICLA + employer authorization, v1.0)
- CLA.zh.md: non-binding Chinese reference translation
- CONTRIBUTING.md: bilingual contributing guide, points to CLA
- .github/workflows/cla.yml: self-hosted cla-assistant-action that records
  signatures into cla-signatures/version-1.json; exempts maintainers & bots
  via allowlist; skips when CLA_BOT_TOKEN is unset
- .github/PULL_REQUEST_TEMPLATE.md: guides contributors to sign
- README.{md,en.md,ja.md}: add License & contributing footer
- app/terms: note CLA requirement in the IP section

Enforcement requires repo-level setup (PAT secret + branch protection)
documented in cla.yml; not covered by this commit.

app/terms/page.tsx

@@ -97,6 +97,24 @@ export default function TermsPage() {
           <p className="mt-3">
             通过本服务生成的游戏内容（包括故事文本、图片和语音）由您在游戏会话期间创造性地引导产生。我们不主张对您个人游戏会话中生成的内容拥有所有权。
           </p>
+          <p className="mt-3">
+            外部贡献者向开源项目提交代码前，需先签署一次《贡献者许可协议》（CLA），明确授予项目维护者将相关贡献用于本服务（含闭源版本）的权利。详见 GitHub 仓库中的{" "}
+            <a
+              href="https://github.com/zonghaoyuan/infiplot/blob/staging/CLA.md"
+              target="_blank"
+              rel="noopener noreferrer"
+              className="text-ember-500 hover:text-ember-400 transition-colors underline decoration-clay-900/20 underline-offset-2"
+            >
+              CLA.md
+            </a>
+            （<a
+              href="https://github.com/zonghaoyuan/infiplot/blob/staging/CLA.zh.md"
+              target="_blank"
+              rel="noopener noreferrer"
+              className="text-ember-500 hover:text-ember-400 transition-colors underline decoration-clay-900/20 underline-offset-2"
+            >
+              中文参考译文</a>）。
+          </p>
         </section>
 
         <section>