feat(adapters): add overriddenBuiltin flag to API and Adapter Manager UI

When an external plugin overrides a built-in adapter type, the
GET /api/adapters response now includes overriddenBuiltin: true. The
Adapter Manager shows an 'Overrides built-in' badge on such adapters.

server/src/routes/adapters.ts

@@ -63,6 +63,8 @@ interface AdapterInfo {
   modelsCount: number;
   loaded: boolean;
   disabled: boolean;
+  /** True when an external plugin has replaced a built-in adapter of the same type. */
+  overriddenBuiltin?: boolean;
   version?: string;
   packageName?: string;
   isLocalPath?: boolean;
@@ -105,6 +107,7 @@ function buildAdapterInfo(adapter: ServerAdapterModule, externalRecord: AdapterP
     modelsCount: (adapter.models ?? []).length,
     loaded: true, // If it's in the registry, it's loaded
     disabled: disabledSet.has(adapter.type),
+    overriddenBuiltin: externalRecord ? BUILTIN_ADAPTER_TYPES.has(adapter.type) : undefined,
     // Prefer on-disk package.json so the UI reflects bumps without relying on store-only fields.
     version: fromDisk ?? externalRecord?.version,
     packageName: externalRecord?.packageName,

ui/src/api/adapters.ts

@@ -17,6 +17,8 @@ export interface AdapterInfo {
   packageName?: string;
   /** Whether the adapter was installed from a local path (vs npm). */
   isLocalPath?: boolean;
+  /** True when an external plugin has replaced a built-in adapter of the same type. */
+  overriddenBuiltin?: boolean;
 }
 
 export interface AdapterInstallResult {

ui/src/pages/AdapterManager.tsx

@@ -63,6 +63,11 @@ function AdapterRow({
               {adapter.label || getAdapterLabel(adapter.type)}
             </span>
             <Badge variant="outline">{adapter.source === "external" ? "External" : "Built-in"}</Badge>
+            {adapter.overriddenBuiltin && (
+              <Badge variant="secondary" className="text-blue-600 border-blue-400">
+                Overrides built-in
+              </Badge>
+            )}
             {adapter.source === "external" && (
               adapter.isLocalPath
                 ? <FolderOpen className="h-4 w-4 text-amber-500" title="Installed from local path" />