fix(errors): add NETWORK exit code to resolve typecheck failure

The handler.ts uses ExitCode.NETWORK but NETWORK was not defined in the
ExitCode enum, causing TypeScript error TS2339 during CI typecheck.

Author: victor0602

src/errors/codes.ts

@@ -5,6 +5,7 @@ export const ExitCode = {
   AUTH: 3,
   QUOTA: 4,
   TIMEOUT: 5,
+  NETWORK: 6,
   CONTENT_FILTER: 10,
 } as const;