config-as-code: migrate comment/audience/public-surface settings off the DB

[JSONbored]

Part of #6440.

Fields: `commentMode`, `publicAudienceMode`, `publicSignalLevel`, `checkRunMode`, `checkRunDetailLevel`, `publicSurface`, `includeMaintainerAuthors`, `badgeEnabled`, `publicQualityMetrics`, `backfillEnabled`, `regateSweepOrderMode`.

All already parse correctly from `.loopover.yml`'s `settings:` block (confirmed via audit — zero silent-discard bugs). This issue is the DB-removal half: backfill each live repo's current effective value into its `.loopover.yml`, stop reading these columns as a merge base in `resolveEffectiveSettings`, drop the DB columns via migration, remove from the dashboard settings write path.

[linear-code]

JSO-4438