Publish structured Join us opportunities

[advaitmb]

Summary

• replace legacy recruitment prose with structured postdoc and PhD opportunity cards
• show status, faculty lead, verification date, timelines, and direct application links
• use a neutral empty state when no opportunities are open
• remove five unrelated legacy recruitment PDFs from another lab
• document and validate the opportunity content model

Validation

• npm test
• npm run test:python
• npm run test:skeleton
• production static export