ci(windows): make build workflow manual

Signed-off-by: Piotr Mlocek <1116309+pimlock@users.noreply.github.com>

.github/workflows/windows-msvc.yml

@@ -1,8 +1,6 @@
 name: Windows MSVC (build-only)
 on:
-  push:
-    branches: [main]
-  pull_request:
+  workflow_dispatch:
 jobs:
   x64:
     runs-on: windows-2025

architecture/windows-msvc-build.md

@@ -48,6 +48,10 @@ creating misleading Windows driver artifacts.
 
 ## Mise Lane
 
+The GitHub Actions workflow is manually dispatched while the clean Windows job
+takes roughly 80 minutes. It should not run for every pull request or merge to
+`main` until the lane has an effective Cargo/native-dependency cache.
+
 Windows validation is exposed through `tasks/windows.toml`:
 
 | Task | Purpose |