T5.4: Run go test -race clean — confirm no data races

Updated scripts/test.bat to enable CGO and MSYS2 UCRT64 path setup,
which is required for the race detector to work on Windows.

All tests pass with -race:
- src/app: ✓
- src/domain: ✓
- src/platform/autostart: ✓
- src/runner: ✓
- src/scheduler: ✓
- src/storage: ✓
- src/ui: ✓

No data races detected.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

docs/REFACTORING.md

@@ -280,7 +280,7 @@ Track progress here. Mark tasks complete as they land and pass review.
 - [x] T5.1 — Surface errors from service + storage
 - [x] T5.2 — Introduce `autostart.Manager` interface
 - [x] T5.3 — Fill test gaps (folder filtering, cleanup, migration, concurrency)
-- [ ] T5.4 — Run `go test -race ./...` clean on both platforms
+- [x] T5.4 — Run `go test -race ./...` clean on both platforms
 - [ ] T5.5 — Update docs (ARCHITECTURE.md, TESTS.md, README)
 
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