test(rules-injector): cover per-session cache isolation and invalidation

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

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

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

@@ -0,0 +1,57 @@
+import { afterEach, describe, expect, it } from "bun:test";
+import { randomUUID } from "node:crypto";
+import { existsSync } from "node:fs";
+import { join } from "node:path";
+import { RULES_INJECTOR_STORAGE } from "./constants";
+import {
+  clearInjectedRules,
+  loadInjectedRules,
+  saveInjectedRules,
+} from "./storage";
+
+const trackedSessionIDs: string[] = [];
+
+function createSessionID(prefix: string): string {
+  const sessionID = `${prefix}-${randomUUID()}`;
+  trackedSessionIDs.push(sessionID);
+  return sessionID;
+}
+
+function getStoragePath(sessionID: string): string {
+  return join(RULES_INJECTOR_STORAGE, `${sessionID}.json`);
+}
+
+afterEach(() => {
+  for (const sessionID of trackedSessionIDs.splice(0)) {
+    clearInjectedRules(sessionID);
+  }
+});
+
+describe("storage", () => {
+  it("reads back only the requested session data from session-scoped files", () => {
+    // given
+    const firstSessionID = createSessionID("storage-first");
+    const secondSessionID = createSessionID("storage-second");
+
+    saveInjectedRules(firstSessionID, {
+      contentHashes: new Set(["hash:first"]),
+      realPaths: new Set(["/tmp/first-rule.md"]),
+    });
+    saveInjectedRules(secondSessionID, {
+      contentHashes: new Set(["hash:second"]),
+      realPaths: new Set(["/tmp/second-rule.md"]),
+    });
+
+    // when
+    const firstLoaded = loadInjectedRules(firstSessionID);
+    const secondLoaded = loadInjectedRules(secondSessionID);
+
+    // then
+    expect(existsSync(getStoragePath(firstSessionID))).toBe(true);
+    expect(existsSync(getStoragePath(secondSessionID))).toBe(true);
+    expect([...firstLoaded.contentHashes]).toEqual(["hash:first"]);
+    expect([...firstLoaded.realPaths]).toEqual(["/tmp/first-rule.md"]);
+    expect([...secondLoaded.contentHashes]).toEqual(["hash:second"]);
+    expect([...secondLoaded.realPaths]).toEqual(["/tmp/second-rule.md"]);
+  });
+});