test(test-isolation): add diagnostic regression test for cross-suite leak

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/hooks/rules-injector/test-isolation.test.ts

@@ -0,0 +1,25 @@
+import { beforeEach, describe, expect, it } from "bun:test";
+import { existsSync, readdirSync } from "node:fs";
+import { RULES_INJECTOR_STORAGE } from "./constants";
+
+function readStorageEntries(): readonly string[] {
+	if (!existsSync(RULES_INJECTOR_STORAGE)) return [];
+	return readdirSync(RULES_INJECTOR_STORAGE);
+}
+
+describe("rules injector test isolation", () => {
+	beforeEach(() => {
+		const leakedEntries = readStorageEntries();
+		expect(leakedEntries).toEqual([]);
+	});
+
+	it("#given the shared test setup runs #then persisted injected-rule state starts empty", () => {
+		// given: test-setup beforeEach has already run
+
+		// when
+		const entries = readStorageEntries();
+
+		// then
+		expect(entries).toEqual([]);
+	});
+});