Remove the Health-Score explainer switch, which switched nothing

The admin panel offered a toggle for the Health-Score explainer and
described it as the ? next to the delta line. That ? had already been
taken off the insights surface a release earlier, and the caption that
replaced it went away entirely when the health score was replaced by
the reference composite. Since then an operator could turn the switch
off, get a saved confirmation, and change absolutely nothing.

The toggle, its wording in all six languages, its place in the flag
matrix the app and the iOS client both read, and its column on
app_settings are all gone. Nothing is left to promise a control that
does not exist.

A guard goes in with it: every remaining assistant sub-flag has to be
read by some surface, and the panel and the plumbing that carry all of
them by construction do not count as proof.

src/components/admin/assistant-section.tsx

@@ -12,11 +12,10 @@ import { SettingsToggle } from "./_shared";
 import { apiGet, apiPut } from "@/lib/api/api-fetch";
 
 /**
- * v1.4.31 — operator-side panel for the six assistant feature
- * flags. The master toggle gates the whole assistant; five
+ * v1.4.31 — operator-side panel for the five assistant feature
+ * flags. The master toggle gates the whole assistant; four
  * sub-toggles carve specific surfaces (Coach, Daily Briefing,
- * per-metric status cards, correlation narration, Health-Score
- * delta explainer).
+ * per-metric status cards, correlation narration).
  *
  * UX:
  *   - Master toggle at the top. When off, the sub-toggles are
@@ -35,7 +34,6 @@ interface AssistantFlagsResponse {
     assistantBriefingEnabled: boolean;
     assistantInsightStatusEnabled: boolean;
     assistantCorrelationsEnabled: boolean;
-    assistantHealthScoreExplainerEnabled: boolean;
   };
   resolved: {
     enabled: boolean;
@@ -43,7 +41,6 @@ interface AssistantFlagsResponse {
     briefing: boolean;
     insightStatus: boolean;
     correlations: boolean;
-    healthScoreExplainer: boolean;
   };
 }
 
@@ -154,17 +151,6 @@ export function AssistantSection() {
             }
             disabled={disabledSubs}
           />
-          <SettingsToggle
-            label={t("admin.assistant.healthScoreExplainer.title")}
-            description={t("admin.assistant.healthScoreExplainer.description")}
-            checked={raw?.assistantHealthScoreExplainerEnabled ?? true}
-            onCheckedChange={(checked) =>
-              mutation.mutate({
-                assistantHealthScoreExplainerEnabled: checked,
-              })
-            }
-            disabled={disabledSubs}
-          />
         </div>
       </div>
     </SettingsCard>