Loses current directory under Windows WSL

I believe the alpha Windows support mentioned in the readme is for WSL, given #105 is asking for native.

When run from WSL (2.7.10) on Win10, the current directory is lost, causing srt to fall back to the Windows directory.

Example below. Same happens when `claude` is replaced with `echo "hi"` - indicating that the path issue is in srt rather than claude.

```
rwilliams:~/test1$ npx @anthropic-ai/sandbox-runtime claude
srt-win: handle fence: skipped (no --holder-pid)
'\\wsl.localhost\Ubuntu\home\rwilliams\test1'
CMD.EXE was started with the above path as the current directory.
UNC paths are not supported. Defaulting to Windows directory.

────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
Accessing workspace:

C:\Windows

Quick safety check: Is this a project you created or one you trust? (Like your own code, a well-known open source
project, or work from your team). If not, take a moment to review what's in this folder first.

Claude Code'll be able to read, edit, and execute files here.

Security guide

❯ 1. Yes, I trust this folder
2. No, exit
```