decK should error out on presence of invalid unicode characters in yaml files

decK fails silently when certain unicode characters replace spaces for indentation - for example, when `u+00a0` is present before `config.*`, `deck file add-plugins` fails silently and sets `config` to `null`.
We should error out explicitly.

Start at the `deck file add-plugins` entry point and trace how its YAML input is parsed when a non-breaking space appears before `config.*`. Reproduce the silent `config: null` result with the example character, then verify that invalid indentation produces an explicit error.

Written by the indexing model from the issue text.