Document Pipeline Builder validation rules

[baixiangcpp]

Summary

• Add Pipeline Builder validation guidance covering checked rules, linear-only execution, repeatable tools, limits, and unsupported handoffs.
• Add an in-page Pipeline Builder usage guide linked from the validation panel.
• Localize the new guidance and extend UI/unit coverage for valid and invalid recipe configurations.

Validation

• npm test -- tests/component/phase3-pipeline-builder-page.test.tsx tests/unit/pipeline-foundation.test.ts
• npm test -- tests/component/representative-a11y-axe.test.tsx
• npm run check:i18n
• npm run check:i18n-qa
• npm run check:types
• npm run lint (passes with existing coverage/block-navigation.js warning)

Closes #254

[cloudflare-workers-and-pages]

Deploying byteflow with    Cloudflare Pages

Latest commit:	5788287
Status:	✅  Deploy successful!
Preview URL:	https://b1fc56cc.byteflow-c58.pages.dev
Branch Preview URL:	https://fix-pipeline-validation-guid.byteflow-c58.pages.dev

View logs