fix(write-existing-file-guard): defer realpath/existsSync to first tool call

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

@@ -59,7 +59,7 @@ describe("createWriteExistingFileGuardHook", () => {
     ).rejects.toThrow("File already exists. Use edit tool instead.")
 
     // then
-    expect(existsSyncMock).toHaveBeenCalledTimes(2)
-    expect(realpathNativeMock).toHaveBeenCalledTimes(1)
+    expect(existsSyncMock).toHaveBeenCalledTimes(3)
+    expect(realpathNativeMock).toHaveBeenCalledTimes(2)
   })
 })