[CI/CD] Cache Turbopack + ESLint on GitHub Actions (halve CI time)

[nanaf6203-bit]

Problem Statement

GitHub Actions runs tsc and next build cold on every PR. Average CI time exceeds 8 minutes.

Acceptance Criteria

• actions/cache@v4 keyed on next.lock/pnpm-lock for Turbopack
• eslint --cache enabled
• Average PR CI drops below 4 minutes

Implementation Notes

Cache directories: .next/cache, .eslintcache. Verify cache hit rates via Actions dashboard.

Files Likely Affected

• .github/workflows/ci.yml
• .eslintignore exclude .next/

Difficulty / Effort

• Difficulty: Easy
• Effort: T-Shirt S

Labels

ci-cd, priority:medium