Fix auto-truncate false setting

[vrdn-23]

What does this PR do?

Summary

The --auto-truncate CLI flag could not be set to false via the command line after its default was changed to true in #829. This is because clap treats bool fields as presence-based flags, so --auto-truncate false would error with "unexpected argument 'false' found".

The fix adds num_args = 0..=1, default_missing_value = "true" to the clap attribute, allowing the flag to optionally accept an explicit value. All existing usage remains backward compatible:

Usage	Result
(flag omitted)	true (default)
--auto-truncate	true (backward compatible)
--auto-truncate true	true
--auto-truncate false	false (new)

The AUTO_TRUNCATE env var continues to work as before.

Fixes #833

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@alvarobartt @kozistr @michaelfeil

[kozistr]

lgtm!