Fix proxy replay across upstream accounts

[Dikevin]

Summary

• rebuild ChatGPT websocket replay requests from proxy checkpoints instead of forwarding stale upstream previous_response_id values
• keep transparent API-key /v1/responses requests pinned to upstream previous_response_id from replaying across API keys
• recreate missing proxy upstream backup directories and document replay boundaries

Verification

• pnpm exec rstest tests/proxy.test.ts --testNamePattern "API-key"
• pnpm exec rstest tests/proxy.test.ts
• pnpm typecheck && pnpm test && pnpm build
• isolated live-run with built dist/cli.js proxy serve, temp HOME/CODEX_HOME, mock upstream: verified previous_response_id skip and normal API-key replay sk-one -> sk-two