Set Bot Repo Config

Merge a per-repo PR-bot config patch into repos.bot_config.

These presentation controls (comment mode, per-section visibility, the score-drop threshold, ignored paths) are the dashboard counterpart to .repowise/bot.yaml and win over it -- the bot overlays them on top of the in-repo YAML at analysis time. The master on/off switch stays on the dedicated /enabled route. Only the keys present in the payload are changed, so the UI can send partial updates. A top-level clear list removes those keys (back to the YAML/default); a key cannot be both cleared and set in one request.