chore(dashboard): drop unused AgentStatus exports on both sides (#1222)

The Phase 5 dead-code sweep across the dashboard call graph found two
identically shaped findings: the AgentStatus interface is declared and
exported in both backend/src/routes/dashboard.ts and
frontend/src/components/dashboard/useConfigurationStatus.ts, but no other
file imports it. (The frontend StackAlertSheet component has a separate,
differently shaped private AgentStatus that does not refer to either of
these.)

Drop the export keyword on both. The interfaces stay alive as
file-internal types, the public surface shrinks by two names, and no
behaviour changes.

The broader payload-cleanup opportunities surfaced during the sweep
(unused requiredTier fields on individual row objects, unused top-level
tier and variant fields on ConfigurationStatus) are out of scope for this
PR and have been filed as Linear roadmap items.

backend/src/routes/dashboard.ts

@@ -6,7 +6,7 @@ import type { LicenseTier, LicenseVariant } from '../services/license-types';
 
 export const dashboardRouter = Router();
 
-export interface AgentStatus {
+interface AgentStatus {
   configured: boolean;
   enabled: boolean;
 }

frontend/src/components/dashboard/useConfigurationStatus.ts

@@ -3,7 +3,7 @@ import { useNodes } from '@/context/NodeContext';
 import { apiFetch } from '@/lib/api';
 import { visibilityInterval } from '@/lib/utils';
 
-export interface AgentStatus {
+interface AgentStatus {
   configured: boolean;
   enabled: boolean;
 }