fix: apply -warnaserror reliably

act_runner mangles boolean input defaults: format('{0}', inputs.warnaserror)
evaluated to '%!t(string=false)', so -warnaserror was never passed.
Input is now a string defaulting to "true"; anything but "false" enables it.
Build command is traced (set -x) so the log shows the flags used.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

README.md

@@ -27,7 +27,7 @@ jobs:
 | `solution` | `""` | Solution/project file, relative to `working-directory`. If empty, dotnet uses the single sln/proj in that directory. |
 | `dotnet-version` | `10.0.x` | .NET SDK version |
 | `configuration` | `Release` | Build configuration |
-| `warnaserror` | `true` | Pass `-warnaserror` to `dotnet build` |
+| `warnaserror` | `"true"` | Pass `-warnaserror` to `dotnet build` unless the value is `"false"` |
 | `working-directory` | `.` | Where dotnet commands run |
 | `test-filter` | `""` | Optional `dotnet test --filter` expression |