Eval sandbox auth is now handed to child processes via file descriptor or snapshot file instead of env vars directly

Eval sandbox subprocesses now receive OAuth and gateway credentials through a new vf() helper instead of directly via environment variables. On non-Windows this uses CLAUDE_CODE_OAUTH_TOKEN_FILE_DESCRIPTOR (or the gateway equivalent); on Windows it writes a snapshot.json under a claude-eval-auth- temp directory and points CLAUDE_BG_AUTH_SNAPSHOT_PATH at it. Stale snapshot directories older than 2 hours are cleaned up automatically on startup.