[Repo Assist] fix(scripts): derive TFM dynamically in validate-wsl-gateway.ps1

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Summary

Fixes validate-wsl-gateway.ps1 to dynamically derive the TargetFramework from OpenClaw.Tray.WinUI.csproj instead of hardcoding net10.0-windows10.0.19041.0.

Closes #432

Root Cause

The script had a hardcoded path:

src\OpenClaw.Tray.WinUI\bin\Debug\net10.0-windows10.0.19041.0\<rid>\OpenClaw.Tray.WinUI.exe

The project now targets net10.0-windows10.0.22621.0, so the executable was never found at the expected path. With -NoBuild, this caused an immediate "executable not found" error; without it, a fresh build would produce output at the new TFM path and the script would still fail to locate it.

Fix

Read TargetFramework from the csproj at startup using PowerShell XML parsing:

$trayTfm = ([xml](Get-Content $trayProject)).Project.PropertyGroup |
    ForEach-Object { $_.TargetFramework } |
    Where-Object { $_ } |
    Select-Object -First 1
if (-not $trayTfm) { throw "Could not derive TargetFramework from $trayProject" }
$trayExe = Join-Path $repoRoot "src\OpenClaw.Tray.WinUI\bin\Debug\$trayTfm\$runtimeIdentifier\OpenClaw.Tray.WinUI.exe"

This also ensures the error message for a missing executable shows the actual path being probed.

Test Status

Script is a PowerShell-only validation helper; no C# test suite covers it directly. The fix was manually verified by confirming the csproj XML parse produces net10.0-windows10.0.22621.0 from the current project file. Build and unit tests are unaffected:

dotnet test ./tests/OpenClaw.Shared.Tests/OpenClaw.Shared.Tests.csproj
dotnet test ./tests/OpenClaw.Tray.Tests/OpenClaw.Tray.Tests.csproj

(Infrastructure-only — runs on Linux, no Windows build change.)

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