The slug-match step treated an entry whose api_backend equals the default
as unset and overwrote it with a same-slug sibling's backend, so
`api_backend = "chat_completions"` silently became messages/responses.
Config keys whose override sets api_backend are recorded and skipped by
the donor step; an unset backend still inherits.
Ported from upstream 1.0.22 (grok-build agent/config.rs
explicit_api_backend_keys).
Claude-Session: https://claude.ai/code/session_01RUqV8HVGXcAETyYbZ2tPQ4
雷电芽衣
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