feat(studio): add migration effect evaluation

[wujiaming-ai]

Summary

• add optional, low-barrier migration effect evaluation with manual and bulk question entry
• persist immutable evaluation datasets and reports independently from migration artifacts
• run bounded evaluation in the existing Dev Sandbox with a fresh Codex thread and temporary AgentKit Runtime
• expose two-stage progress, evidence-aware scoring, failures, cancellation, retry, and report download

Verification

• npm test — 1042 passed
• migration/evaluation backend suite — 186 passed
• TypeScript, i18n, and production frontend build passed
• Ruff check/format and focused Pyright passed
• pre-commit passed, including secret scanning
• packaged WebUI verifier passed: 102 files / 246 internal references
• browser visual checks reviewed at 1280px and 390px