- Add StdinGuard for safe single-read stdin across multiple flags
- Add ResolveInputSource unified resolver for @file, @-, and pipe
- Support @file / @- syntax for all string-typed override flags
- Remove unused writeSchemaText function
- Add stdin.go with ReadStdinIfPiped and ReadFileArg helpers
- Support @file syntax for --json flag to read payload from file
- Fall back to stdin pipe when no --json argument is provided