agent: address argus review notes — stale comment + CI typecheck wiring

- StatusChips.tsx: drop the stale 'useAgentState lands in a later
  commit' comment now that Root.tsx already wires it.
- nix/checks.nix: add agent-typecheck, same shape as swarm-ui-typecheck,
  so a real TS error in the agent package fails CI instead of just
  esbuild-stripping silently.

frontend/packages/agent/src/components/StatusChips.tsx

@@ -6,9 +6,9 @@
 // badge here IS the control for what it displays, composed from
 // `@hive/shared`'s `Badge`/`Dropdown` (../../shared/src/badge,
 // ../../shared/src/dropdown). Presentational only: formatting +
-// selection state live in the caller (a `useAgentState` hook lands in a
-// later commit), so this component can be demoed and reviewed against
-// plain sample data before it's wired to `/api/state` polling.
+// selection state live in the caller (`Root.tsx`, via the
+// `useAgentState` hook), so this component can still be demoed and
+// reviewed against plain sample data independent of live `/api/state`.
 import { useState } from 'preact/hooks';
 import { Badge, type BadgeTone } from '@hive/shared/badge.js';
 import { Dropdown, type DropdownOption } from '@hive/shared/dropdown.js';