fix(auth): prevent credential resurrection after Anthropic logout

The logout flow had two bugs causing credentials to reappear immediately:

1. The codebase has two separate auth storage Proxy chains:
   - createFusionAuthStorage (engine, for agents)
   - mergeAuthStorageReads (CLI, for dashboard UI)
   Neither had a logout trap, so supplemental credentials from
   ~/.claude/.credentials.json were never excluded after logout.

2. The upstream AuthStorage.hasAuth() checks environment variables
   (ANTHROPIC_API_KEY), which always returns true regardless of logout.

Fix: Add loggedOutProviders tracking to both Proxy chains. All query
traps (has, hasAuth, get, getAll, list, getApiKey) return false/undefined
for logged-out providers instead of delegating to the underlying storage.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/dashboard/src/routes/register-auth-routes.ts

@@ -899,6 +899,7 @@ export const registerAuthRoutes: ApiRouteRegistrar = (ctx) => {
 
       const storage = getAuthStorage();
       storage.logout(provider);
+      clearUsageCache();
       res.json({ success: true });
     } catch (err: unknown) {
       if (err instanceof ApiError) {