Replace `read failed: {e}` / `parse failed: {e}` with the underscore-
slugged `read_failed:` / `parse_failed:` prefixes so kebab-cli's
error_classify (Task 8) and the error.v1 JSON Schema (Task 14) can
treat the prefix as a stable wire contract while leaving the OS /
toml-crate detail in the suffix as free-form context.
Also add the symmetric `cause` non-empty assertion to the malformed-TOML
test so a regression that empties `cause` on the parse path would be
caught.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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