docs: correct directory JSON failure envelope (#124989)

docs: correct directory JSON failure envelope (#124989)

* docs: correct directory JSON failure envelope

* docs(cli): document canonical config and directory JSON failures

Preserve the contributor directory error-envelope correction and update config get to distinguish schema-valid unset paths from unknown paths while linking the shared canonical CLI JSON failure contract.

Co-authored-by: qingminlong 

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Co-authored-by: Peter Steinberger