chore: drop stale references to the onboarding offer and old resolver

The Playwright project list still named the deleted onboarding AI
offer spec. The CoachPlan comments said the review writes
`outcomeEncrypted` and surfaces it behind a flag; nothing writes or
reads it now, and the column is marked internal. Comments that named
`resolveProvider` as the resolver now name the chain, and the function
itself says it is test-only.

src/components/settings/ai/shared.ts

@@ -127,8 +127,8 @@ export const LOCAL_MODEL_PRESETS = [
 export const CUSTOM_MODEL_SENTINEL = "__custom__";
 
 /**
- * Map UI provider tag → the `aiProvider` enum understood by the
- * legacy single-result `resolveProvider()` resolver. `codex` and
+ * Map UI provider tag → the stored `aiProvider` enum, the provider picked
+ * under Settings, AI. `codex` and
  * `admin-openai` aren't user-level provider columns (they're handled
  * by Codex OAuth + admin-key fallback), so they map to null and the
  * legacy column is left blank.