Remove legacy venv Scripts entry from User PATH on upgrade

Older installers persisted the venv Scripts directory directly in the
User PATH registry. The shim approach (added in this PR) no longer writes
that entry, but it also did not remove the old one. On upgrade, the
legacy entry survived and python.exe / pip.exe from the unsloth venv
continued winning resolution in every new shell, which is exactly the
hijack the shim was designed to prevent.

Before creating the shim, read the current User PATH, filter out any
entry matching $VenvDir\Scripts (using the same symmetric raw+expanded
comparison as Add-ToUserPath), and write back if changed. This runs
once per install and is a no-op on fresh installs where the legacy
entry was never written.

install.ps1

@@ -1042,6 +1042,41 @@ shell.Run cmd, 0, False
     # We do NOT add the venv Scripts dir to PATH (it also holds python.exe
     # and pip.exe, which would hijack the user's system interpreter).
     # Hardlink preferred; falls back to copy if cross-volume or non-NTFS.
+    #
+    # Clean up the legacy venv Scripts PATH entry that older installers wrote.
+    # Without this, upgrade users keep python/pip hijacked even after the shim
+    # approach is in place.
+    $LegacyScriptsDir = Join-Path $VenvDir "Scripts"
+    try {
+        $legacyKey = [Microsoft.Win32.Registry]::CurrentUser.CreateSubKey('Environment')
+        try {
+            $rawPath = $legacyKey.GetValue('Path', '', [Microsoft.Win32.RegistryValueOptions]::DoNotExpandEnvironmentNames)
+            if ($rawPath) {
+                [string[]]$pathEntries = $rawPath -split ';'
+                $normalLegacy = $LegacyScriptsDir.Trim().Trim('"').TrimEnd('\').ToLowerInvariant()
+                $expNormalLegacy = [Environment]::ExpandEnvironmentVariables($LegacyScriptsDir).Trim().Trim('"').TrimEnd('\').ToLowerInvariant()
+                $filtered = @($pathEntries | Where-Object {
+                    $stripped = $_.Trim().Trim('"')
+                    $rawNorm = $stripped.TrimEnd('\').ToLowerInvariant()
+                    $expNorm = [Environment]::ExpandEnvironmentVariables($stripped).TrimEnd('\').ToLowerInvariant()
+                    ($rawNorm -ne $normalLegacy -and $rawNorm -ne $expNormalLegacy) -and
+                    ($expNorm -ne $normalLegacy -and $expNorm -ne $expNormalLegacy)
+                })
+                $cleanedPath = $filtered -join ';'
+                if ($cleanedPath -ne $rawPath) {
+                    $legacyKey.SetValue('Path', $cleanedPath, [Microsoft.Win32.RegistryValueKind]::ExpandString)
+                    # Broadcast so other processes see the removal immediately.
+                    try {
+                        $d = "UnslothPathRefresh_$([guid]::NewGuid().ToString('N').Substring(0,8))"
+                        [Environment]::SetEnvironmentVariable($d, '1', 'User')
+                        [Environment]::SetEnvironmentVariable($d, [NullString]::Value, 'User')
+                    } catch { }
+                }
+            }
+        } finally {
+            $legacyKey.Close()
+        }
+    } catch { }
     $ShimDir = Join-Path $StudioHome "bin"
     New-Item -ItemType Directory -Force -Path $ShimDir | Out-Null
     $ShimExe = Join-Path $ShimDir "unsloth.exe"