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/BudgetAlertBanner.tsx

@@ -0,0 +1,123 @@
+import { useState, useEffect, useMemo, useCallback } from "react";
+import { Warning, X } from "@phosphor-icons/react";
+import { AnimatePresence, motion } from "framer-motion";
+import type { AgentRecord, SectionId } from "@shared/types";
+import { useReducedMotion } from "../ui";
+
+export interface BudgetAlertBannerProps {
+  agents: AgentRecord[];
+  onNavigate: (section: SectionId, entityId?: string) => void;
+}
+
+interface AgentOverThreshold {
+  id: string;
+  name: string;
+  pct: number;
+}
+
+export function BudgetAlertBanner({ agents, onNavigate }: BudgetAlertBannerProps) {
+  const reducedMotion = useReducedMotion();
+  const [dismissed, setDismissed] = useState(false);
+
+  const agentsOverThreshold = useMemo<AgentOverThreshold[]>(() => {
+    return agents
+      .filter((a) => a.budgetMonthlyUsd > 0 && a.spentMonthlyUsd / a.budgetMonthlyUsd >= 0.8)
+      .map((a) => ({
+        id: a.id,
+        name: a.name,
+        pct: Math.round((a.spentMonthlyUsd / a.budgetMonthlyUsd) * 100),
+      }))
+      .sort((a, b) => b.pct - a.pct);
+  }, [agents]);
+
+  // Reset dismissed state when the set of agents over threshold changes
+  const overThresholdKey = useMemo(
+    () => agentsOverThreshold.map((a) => a.id).sort().join(","),
+    [agentsOverThreshold],
+  );
+
+  useEffect(() => {
+    setDismissed(false);
+  }, [overThresholdKey]);
+
+  const handleDismiss = useCallback(() => setDismissed(true), []);
+  const handleViewCosts = useCallback(() => onNavigate("costs"), [onNavigate]);
+
+  if (agentsOverThreshold.length === 0) return null;
+
+  const content = (
+    <div
+      role="alert"
+      aria-live="assertive"
+      className="rounded-[10px] border border-[color:var(--warn)] border-opacity-30 bg-[color:var(--warn-soft)] px-4 py-3"
+    >
+      <div className="flex items-start gap-3">
+        {/* Icon */}
+        <div className="mt-0.5 shrink-0">
+          <Warning size={18} weight="bold" className="text-[color:var(--warn)]" />
+        </div>
+
+        {/* Content */}
+        <div className="min-w-0 flex-1">
+          <div className="text-[13px] font-semibold text-[color:var(--warn)]">
+            Budget Alert
+          </div>
+          <div className="mt-0.5 text-[13px] text-[color:var(--muted-strong)]">
+            {agentsOverThreshold.length} agent(s) approaching budget limit
+          </div>
+          <div className="mt-1.5 flex flex-wrap items-center gap-x-3 gap-y-1">
+            {agentsOverThreshold.map((agent) => (
+              <span
+                key={agent.id}
+                className={`text-[11px] font-[family-name:var(--font-mono)] ${
+                  agent.pct >= 100
+                    ? "text-[color:var(--danger)]"
+                    : "text-[color:var(--warn)]"
+                }`}
+              >
+                {agent.name} ({agent.pct}%)
+              </span>
+            ))}
+          </div>
+          <button
+            type="button"
+            onClick={handleViewCosts}
+            className="mt-1.5 text-[11px] font-medium text-[color:var(--accent)] hover:underline cursor-pointer"
+          >
+            View Costs
+          </button>
+        </div>
+
+        {/* Dismiss button */}
+        <button
+          type="button"
+          onClick={handleDismiss}
+          title="Dismiss budget alert"
+          aria-label="Dismiss budget alert"
+          className="shrink-0 rounded p-0.5 text-[color:var(--muted-strong)] transition hover:text-[color:var(--text)]"
+        >
+          <X size={14} />
+        </button>
+      </div>
+    </div>
+  );
+
+  if (dismissed) return null;
+
+  if (reducedMotion) {
+    return content;
+  }
+
+  return (
+    <AnimatePresence>
+      <motion.div
+        initial={{ opacity: 0, y: -8 }}
+        animate={{ opacity: 1, y: 0 }}
+        exit={{ opacity: 0, y: -8 }}
+        transition={{ duration: 0.2, ease: "easeOut" }}
+      >
+        {content}
+      </motion.div>
+    </AnimatePresence>
+  );
+}

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">