Package banner holds the txco logo + small TTY helpers used by
every subcommand's help/usage screen. Lives in its own package so
both chassis/cli and chassis/cli/auth can import it without a cycle
(chassis/cli imports chassis/cli/auth for dispatch).
IsTTY reports whether w wraps a character device (i.e. an
interactive terminal). Returns false for pipes, files, and any
non-*os.File writer. No external dep required — ModeCharDevice on
the FileInfo is the same check mattn/go-isatty performs on
darwin/linux.
PrintLogo writes the txco banner to w. The three dots colorize
(cyan/magenta/yellow) when w is a terminal; piped output stays
plain so it doesn't leak escape codes into log files or pagers.