feat: display credits balance in list - #155

Summary

• Display credits.balance in a dedicated CREDITS column for codex-auth list.
• Keep RESET CREDITS unchanged as a separate value.
• Preserve numeric usage responses and display the integer portion without rounding.

Validation

• API parsing tests: 9/9 passed.
• Table tests and zig build run -- list are blocked locally by a Zig 0.16.0 Windows ARM64 translate-c.exe failure while translating C system headers (time.h).