[FT] Store system prompt in results

[lewtun]

Issue encountered

Currently, when one provides a system prompt at runtime, the system prompt is only visible in the full_prompt column of the details. It would be great if this could be propagated to the results file as a config_general.system_prompt field.

Solution/Feature

Propagate the system prompt at runtime to a config_general.system_prompt field in the results file, and leave as an empty string by default

Possible alternatives

Currently I workaround this issue as follows:

jq --arg prompt "$SYSTEM_PROMPT" '.config_general.system_prompt = $prompt' "$filepath" > "${filepath}.tmp" && mv "${filepath}.tmp" "$filepath"

[NathanHB]

The refacto PR #709 Just made this the case by moving the --system-prompt to the model config, whichi is then logged in the results file