test(rules-injector): cover project-root-finder memoization

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

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

src/hooks/rules-injector/project-root-finder.test.ts

@@ -0,0 +1,47 @@
+import { afterEach, describe, expect, it, mock } from "bun:test";
+
+describe("findProjectRoot", () => {
+  afterEach(async () => {
+    const actualFileSystem = await import("node:fs");
+    mock.module("node:fs", () => actualFileSystem);
+  });
+
+  it("memoizes repeated lookups for the same start path and resets on cache clear", async () => {
+    // given
+    const actualFileSystem = await import("node:fs");
+    const projectRoot = "/workspace/project";
+    const startPath = `${projectRoot}/src/file.ts`;
+    const packageJsonPath = `${projectRoot}/package.json`;
+
+    const existsSyncSpy = mock((path: string) => path === packageJsonPath);
+    const statSyncSpy = mock(() => ({ isDirectory: () => false }));
+
+    mock.module("node:fs", () => ({
+      ...actualFileSystem,
+      existsSync: existsSyncSpy,
+      statSync: statSyncSpy,
+    }));
+
+    const { clearProjectRootCache, findProjectRoot } = await import(
+      `./project-root-finder.ts?memoization=${Date.now()}`
+    );
+
+    // when
+    const firstResult = findProjectRoot(startPath);
+    const firstExistsSyncCallCount = existsSyncSpy.mock.calls.length;
+
+    const secondResult = findProjectRoot(startPath);
+    const secondExistsSyncCallCount = existsSyncSpy.mock.calls.length;
+
+    clearProjectRootCache();
+    const thirdResult = findProjectRoot(startPath);
+
+    // then
+    expect(firstResult).toBe(projectRoot);
+    expect(secondResult).toBe(projectRoot);
+    expect(thirdResult).toBe(projectRoot);
+    expect(firstExistsSyncCallCount).toBeGreaterThan(0);
+    expect(secondExistsSyncCallCount).toBe(firstExistsSyncCallCount);
+    expect(existsSyncSpy).toHaveBeenCalledTimes(firstExistsSyncCallCount * 2);
+  });
+});