refactor(FN-5633): AI merge reviewer uses the project reviewer/validator model

Drop the bespoke `merger.reviewerModel` setting. The AI merge reviewer now
resolves its model via `resolveValidatorSettingsModel` — the same
reviewer/validator lane used elsewhere (project validator → global validator →
project default) — falling back to the merger model only if that lane is empty.
Removes the setting from the type, schema default, and the Settings UI.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/core/src/settings-schema.ts

@@ -262,7 +262,7 @@ export const DEFAULT_PROJECT_SETTINGS = {
   ],
   prerebaseDivergenceThreshold: 50,
   mergeConflictStrategy: "smart-prefer-main",
-  merger: { mode: "ai", reviewerModel: undefined, maxReviewPasses: 3 },
+  merger: { mode: "ai", maxReviewPasses: 3 },
   mergeDiffVolumeMinLines: undefined,
   mergeDiffVolumeThreshold: undefined,
   mergeDiffVolumeAllowlist: undefined,