[codex] Harden extension form mapping

[leini8891]

Summary

• Extracted extension question-to-answer mapping into a pure, unit-testable mapper for preference-backed answers.
• Routed LinkedIn and MyCareersFuture uncertain/unsupported required questions back to manual review instead of guessing.
• Added Greenhouse standard application form support for assisted filling and review handoff.

Newly Supported Preference-Backed Fields

• Work authorization
• Visa sponsorship
• Relocation willingness
• Notice period
• Years of experience
• Expected salary and salary currency

Newly Covered Site

• Greenhouse standard job boards (boards.greenhouse.io, job-boards.greenhouse.io)

Safety Notes

• Answers are sourced only from saved user preferences.
• Ambiguous, unsupported, or sensitive questions such as current salary and exact start date stay blank and enter human review.
• No paid third-party automation services were added or reintroduced.

Tests

• pnpm lint && pnpm test && pnpm build
• pnpm test:e2e

Stacked on #5 / codex/supabase-auth-multiuser to keep this PR focused on the extension mapping and Greenhouse coverage changes.