bin/verify-documented-usage.sh is not environment agnostic

[Xophmeister]

Describe the bug
The bin/verify-documented-usage.sh script checks that the usage documentation in the README matches the various Topiary CLI subcommand --help outputs. However, it does not account for line wrapping in the --help outputs, which appear to be environment-dependent. (My suspicion is the TTY of the process, if there is one.)

This leads to false negatives.

To Reproduce
In the nix develop dev-shell, run verify-documented-usage. It will probably complain that the README is out-of-sync with at least one of the subcommands' --help output. (If not, resize your terminal until it does!)

Expected behaviour
The output produced by the script should be wrapped at a set width (80 columns, say), so it's deterministic.

Environment

• OS name + version: NixOS 24.11
• Version of the code: d61664f

[Xophmeister]

#854 was an abortive attempt to solve this:

• Naively setting $COLUMNS doesn't do anything.
• Using stty to set the terminal width doesn't work because the CI doesn't run in a TTY.
• Using clap::Command::max_term_width didn't work as expected and, even if it did, it's not a satisfactory solution.

I believe the way to do this is to create a pseudoterminal, with a set size, then run the --help commands in that. This cannot be done in a simple Bash script.