Document contributor quality gates

Contributors need a visible path for local verification instead of relying on design notes or maintainer memory. Add a GitHub-facing CONTRIBUTING entrypoint, bilingual docs pages, README links, and VitePress sidebar navigation for PR gates, live model baselines, provider selection, reports, and release checks.

Constraint: Live baseline providers are local machine state; docs must tell contributors how to list and choose their own providers without maintainer UUIDs.

Rejected: Keep the instructions only in the quality-gate design doc | too hidden for clone-and-contribute workflows

Confidence: high

Scope-risk: narrow

Directive: Keep quality-gate commands documented wherever contributor onboarding links are exposed.

Tested: bun run check:docs

Tested: bun run quality:pr

Not-tested: live baseline after the docs-only wording change

docs/.vitepress/config.mts

@@ -24,6 +24,7 @@ const zhSidebar = [
       { text: '第三方模型', link: '/guide/third-party-models' },
       { text: '全局使用', link: '/guide/global-usage' },
       { text: '常见问题', link: '/guide/faq' },
+      { text: '贡献与质量门禁', link: '/guide/contributing' },
     ],
   },
   {
@@ -109,6 +110,7 @@ const enSidebar = [
       { text: 'Third-Party Models', link: '/en/guide/third-party-models' },
       { text: 'Global Usage', link: '/en/guide/global-usage' },
       { text: 'FAQ', link: '/en/guide/faq' },
+      { text: 'Contributing', link: '/en/guide/contributing' },
     ],
   },
   {