feat(coding-agent): merge consecutive cache warming notices into one line

Back-to-back cache warms update a single 'Cache warmed: $<total>' line with the cumulative cost instead of adding one line per warm. Each warm is still persisted as its own usage entry.

packages/coding-agent/test/cache-warmer.test.ts

@@ -338,7 +338,9 @@ describe("cache warming", () => {
 			usage,
 			"extension override",
 		);
-		expect(formatCacheWarmingUsage(entry)).toBe("Cache warmed (extension override): $0.029497");
+		expect(formatCacheWarmingUsage(entry.usage.cost.total, entry.note)).toBe(
+			"Cache warmed (extension override): $0.029497",
+		);
 	});
 });