Allow disabling color

Support `--color=auto/always/never`, where `auto` is the default.

If auto:
* if `$NO_COLOR` is set, `never` ([ref](https://no-color.org/))
* if `$CI` is set, `always`
* if not `hSupportsANSI`, `never`
* otherwise, `always`

To ensure we catch everything, probably add a global `IORef` in `Term.hs`, set it in `Main.hs`, and in the color functions, check this value

Start by reading Main.hs, Term.hs, and the existing color functions to trace how command-line options and terminal color output are handled. Implement the specified --color modes and auto-selection rules, then verify that all color output respects the selected mode and that auto remains the default.

Written by the indexing model from the issue text.