Windows cross-drive project-root traversal fix

### What happened?

On Windows, `findProjectRoot()` can enter a synchronous infinite loop when Senpi is launched from one drive and a rules lookup targets a path on another drive.

`resolve("/")` points to the current working drive, while `dirname()` of the target drive root returns the same path. The traversal therefore never progresses or reaches its existing root check. The session freezes without an error, potentially using 100% CPU.

This reproduced with `senpi -ne`, so it was not caused by a user-loaded extension.

### Steps to reproduce

1. On Windows, launch Senpi from the `C:` drive:

`senpi -ne`

2. Read, edit, or write a file on another drive, such as `D:\senpi-repro\file.txt`, where no project marker exists in its parent directories.
3. Observe that the session hangs indefinitely during project-root discovery.

### Expected behavior

Project-root traversal should stop when `dirname(currentDirectory) === currentDirectory` and return `null` if no marker was found:

```ts
const parentDirectory = dirname(currentDirectory);

if (
currentDirectory === filesystemRoot ||
parentDirectory === currentDirectory
) {
return null;
}

### Version

2026.7.29-6

Locate the TypeScript implementation of findProjectRoot and trace its traversal from the cross-drive reproduction described above. Verify the parent-directory stop condition on Windows, then add or update coverage for a target drive with no project marker and confirm the lookup returns null instead of hanging.

Written by the indexing model from the issue text.