fix: start-local-agent.ps1 crashes on Windows

Start-Process rejected -RedirectStandardOutput/-RedirectStandardError
pointing at the same file ('inputs must be different'), so on Windows
the launcher always threw before spawning the agent — start.bat,
login, and the desktop console all appeared broken.

Let the child PowerShell redirect its own streams via '*>> $LogFile'
and drop the Start-Process redirection flags entirely.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ouyyyyang

scripts/dev/start-local-agent.ps1

@@ -201,16 +201,14 @@ $serverCommand = @(
     "`$env:SCREEN_PILOT_PID_FILE='$PidFile'"
     "`$env:SCREEN_PILOT_PORT_FILE='$PortFile'"
     "Set-Location '$RootDir'"
-    "& '$nodeAbsolute' build/node/core/agent/src/server.js"
+    "& '$nodeAbsolute' build/node/core/agent/src/server.js *>> '$LogFile'"
 ) -join "; "
 
 $process = Start-Process `
     -FilePath "powershell.exe" `
     -ArgumentList @("-NoProfile", "-ExecutionPolicy", "Bypass", "-Command", $serverCommand) `
     -WorkingDirectory $RootDir `
     -WindowStyle Hidden `
-    -RedirectStandardOutput $LogFile `
-    -RedirectStandardError $LogFile `
     -PassThru
 
 Set-Content -Path $PidFile -Value $process.Id