test(write-existing-file-guard): cover lazy canonical path init

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

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

src/hooks/write-existing-file-guard/lazy-canonical-path-init.test.ts

@@ -0,0 +1,65 @@
+import { afterEach, beforeEach, describe, expect, mock, test } from "bun:test"
+import { mkdirSync, mkdtempSync, rmSync, writeFileSync } from "node:fs"
+import { tmpdir } from "node:os"
+import { join } from "node:path"
+
+const realFs = await import("node:fs")
+
+const existsSyncMock = mock(realFs.existsSync)
+const realpathNativeMock = mock(realFs.realpathSync.native)
+
+mock.module("fs", () => ({
+  ...realFs,
+  existsSync: existsSyncMock,
+  realpathSync: {
+    ...realFs.realpathSync,
+    native: realpathNativeMock,
+  },
+}))
+
+const { createWriteExistingFileGuardHook } = await import("./index")
+
+describe("createWriteExistingFileGuardHook", () => {
+  let tempDir = ""
+
+  beforeEach(() => {
+    // given
+    tempDir = mkdtempSync(join(tmpdir(), "write-existing-file-guard-lazy-"))
+    mkdirSync(tempDir, { recursive: true })
+    existsSyncMock.mockClear()
+    realpathNativeMock.mockClear()
+  })
+
+  afterEach(() => {
+    rmSync(tempDir, { recursive: true, force: true })
+  })
+
+  test("#given hook factory #when created #then defers fs canonical path calls until first tool invocation", async () => {
+    // given
+    const existingFile = join(tempDir, "existing.txt")
+    writeFileSync(existingFile, "content")
+
+    // when
+    const hook = createWriteExistingFileGuardHook({ directory: tempDir } as never)
+
+    // then
+    expect(existsSyncMock).toHaveBeenCalledTimes(0)
+    expect(realpathNativeMock).toHaveBeenCalledTimes(0)
+
+    // when
+    await expect(
+      hook["tool.execute.before"]?.(
+        {
+          tool: "write",
+          sessionID: "ses_lazy",
+          callID: "call_lazy",
+        } as never,
+        { args: { filePath: existingFile, content: "updated" } } as never,
+      ),
+    ).rejects.toThrow("File already exists. Use edit tool instead.")
+
+    // then
+    expect(existsSyncMock).toHaveBeenCalledTimes(2)
+    expect(realpathNativeMock).toHaveBeenCalledTimes(1)
+  })
+})