fix(docs): use dotenv code block lang to unblock Vercel build

Shiki's default bundle doesn't include the `env` grammar, so MDX
prerendering fails with `Language `env` is not included in this
bundle.` The two pages added in #2474 used ```env, which broke both
Preview and Production deployments of multica-docs.

Swap the language tag to `dotenv` (Shiki ships it by default) — same
visual result, no Shiki config change needed.

Refs MUL-2122

Co-authored-by: multica-agent <github@multica.ai>

apps/docs/content/docs/github-integration.mdx

@@ -111,7 +111,7 @@ After **Create GitHub App**, note two things from the App's detail page:
 
 On the API server:
 
-```env
+```dotenv
 GITHUB_APP_SLUG=multica-acme
 GITHUB_WEBHOOK_SECRET=<the webhook secret you generated>
 ```

apps/docs/content/docs/github-integration.zh.mdx

@@ -111,7 +111,7 @@ Self-Host 需要：建一个 GitHub App、指向你的 server、设两个环境
 
 API server 上：
 
-```env
+```dotenv
 GITHUB_APP_SLUG=multica-acme
 GITHUB_WEBHOOK_SECRET=<你刚生成的 webhook secret>
 ```