Get master_key from yaml instead of env vars in benchmarking scripts

Less keys in bash history etc.

Ref `bin/llmao-smoke` and `bin/llmao-saturate`

Start by reading bin/llmao-smoke and bin/llmao-saturate to find how master_key is currently obtained, then inspect the repository’s YAML configuration handling. The work is done when both benchmarking scripts obtain master_key from YAML rather than environment variables and no longer expose it through the shell environment; run the relevant scripts or checks to verify this.

Written by the indexing model from the issue text.