fix(ai): ranked antigravity bottleneck counters first

The google-antigravity ranking strategy originally returned the most-pressured
counter as primary and the runner-up as secondary. AuthStorage compares the
secondary ranking metrics before primary, so credentials with a dangerous
bottleneck but a healthy runner-up counter could outrank credentials with
balanced headroom.

Return the bottleneck as secondary and the runner-up as primary, and update the
strategy contract test to lock the ordering invariant.

Fixes #2187

packages/ai/src/usage/google-antigravity.ts

@@ -312,9 +312,14 @@ const ANTIGRAVITY_DAILY_WINDOW_MS = 24 * 60 * 60 * 1000;
  * Anthropic / OpenAI) per tier per window, and {@link fetchAntigravityUsage}
  * sorts them ascending by `remainingFraction` — so `limits[0]` is always the
  * most-pressured counter for the credential, and `limits[1]` (when present)
- * is the next-most-pressured. Mapping those to {primary, secondary} lets the
- * ranker compare two credentials on both their bottleneck counter and their
- * runner-up before falling back to round-robin.
+ * is the next-most-pressured counter.
+ *
+ * `AuthStorage` compares the `secondary*` ranking metrics before `primary*`
+ * because other providers model a long-window budget as secondary. Antigravity
+ * does not expose a short/long split; every counter is a sibling bottleneck.
+ * Therefore the most-pressured counter goes in `secondary`, with the runner-up
+ * in `primary`, so proactive account selection always ranks the bottleneck
+ * before any healthier sibling counter.
  *
  * The Antigravity API exposes `resetTime` but not window duration, so the
  * drain-rate calculation depends on `windowDefaults`. Antigravity quotas are
@@ -324,7 +329,7 @@ const ANTIGRAVITY_DAILY_WINDOW_MS = 24 * 60 * 60 * 1000;
  */
 export const antigravityRankingStrategy: CredentialRankingStrategy = {
 	findWindowLimits(report) {
-		return { primary: report.limits[0], secondary: report.limits[1] };
+		return { primary: report.limits[1], secondary: report.limits[0] };
 	},
 	windowDefaults: {
 		primaryMs: ANTIGRAVITY_DAILY_WINDOW_MS,

packages/ai/test/google-antigravity-usage.test.ts

@@ -257,18 +257,20 @@ describe("antigravity ranking strategy", () => {
 		};
 	}
 
-	it("maps the most-pressured counter to primary and the runner-up to secondary", () => {
+	it("maps the most-pressured counter to secondary because AuthStorage compares secondary first", () => {
 		// fetchAntigravityUsage sorts ascending by remainingFraction, so a real
-		// report's limits[0] is always the bottleneck — the ranking strategy
-		// MUST surface that as the primary signal, not the first model alphabetically.
+		// report's limits[0] is always the bottleneck. AuthStorage compares the
+		// secondary ranking metrics before primary, so Antigravity must put the
+		// bottleneck there; otherwise [5%, 90%] remaining can beat [40%, 40%]
+		// because the runner-up counter looks healthier.
 		const report = {
 			provider: "google-antigravity" as const,
 			fetchedAt: Date.now(),
 			limits: [makeLimit(0.05, "Anthropic"), makeLimit(0.4, "Google"), makeLimit(0.9, "OpenAI")],
 		};
 		const { primary, secondary } = antigravityRankingStrategy.findWindowLimits(report);
-		expect(primary?.label).toBe("Anthropic");
-		expect(secondary?.label).toBe("Google");
+		expect(secondary?.label).toBe("Anthropic");
+		expect(primary?.label).toBe("Google");
 	});
 
 	it("returns undefined windows when the credential has no usage limits", () => {