feat(03-03): add BudgetAlertBanner component and integrate into Dashboard

- Create BudgetAlertBanner with threshold detection (>=80% budget utilization)
- Display agent names with percentages, warn color for 80-99%, danger for 100%+
- Dismissible per session with auto-reset on new threshold crossings
- View Costs link navigates to costs section
- Accessible with role=alert, aria-live=assertive, keyboard-dismissible
- Framer Motion fade-in/slide-down animation, respects reduced motion
- Positioned after RecoveryBanner in Dashboard layout

src/renderer/components/dashboard/Dashboard.tsx

@@ -43,6 +43,7 @@ import { SprintProgress } from "./SprintProgress";
 import { GoalProgress } from "./GoalProgress";
 import { NeedsAttention } from "./NeedsAttention";
 import { RecoveryBanner } from "./RecoveryBanner";
+import { BudgetAlertBanner } from "./BudgetAlertBanner";
 
 export interface DashboardProps {
   agents: AgentRecord[];
@@ -90,6 +91,9 @@ export function Dashboard(props: DashboardProps) {
       {/* Recovery banner for interrupted runs */}
       <RecoveryBanner />
 
+      {/* Budget alert banner for agents approaching budget limits */}
+      <BudgetAlertBanner agents={agents} onNavigate={onNavigate} />
+
       {/* Health + Metrics */}
       <div className="flex flex-col gap-6 lg:flex-row lg:items-start">
         <div className="shrink-0 rounded-[12px] border border-[color:var(--line)] bg-[color:var(--panel)] p-5">