fix: resolve full PowerShell path on Windows

[elis132]

Problem

The Shell tool is completely broken on Windows when installed via uv tool install. Every shell command fails with:

[WinError 2] The system cannot find the file specified

environment.py sets shell_path = KaosPath("powershell.exe") — a bare filename. When asyncio.create_subprocess_exec() tries to launch it, CreateProcessW cannot find powershell.exe because the uv virtual environment PATH does not include C:\Windows\System32\WindowsPowerShell\v1.0\.

Fix

Resolve the full path using the SystemRoot environment variable (always set on Windows):

system_root = os.environ.get("SystemRoot", r"C:\Windows")
shell_path = KaosPath(
    os.path.join(system_root, "System32", "WindowsPowerShell", "v1.0", "powershell.exe")
)

Verification

import asyncio

async def test():
    # FAILS - bare name
    await asyncio.create_subprocess_exec("powershell.exe", "-command", "echo hello", ...)
    # [WinError 2]

    # WORKS - full path
    await asyncio.create_subprocess_exec(
        r"C:\Windows\System32\WindowsPowerShell\v1.0\powershell.exe", "-command", "echo hello", ...
    )

asyncio.run(test())

Fixes #1702

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