P7.1: move Windows-only runner tests into runner_windows_test.go

Tests touching SysProcAttr.HideWindow/CmdLine and windowsShellCommandLine
are guarded by //go:build windows; runner_test.go drops the winproc import.
Linux cross-compile and Windows test run both clean.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/PRE-RELEASE-TASKS.md

@@ -114,7 +114,7 @@ These land together because both edit `domain/job.go` and `storage/store.go`.
 - [x] P6.3 — Re-measure startup → PERFORMANCE.md
 
 ### Phase 7 — Roadmap follow-ups
-- [ ] P7.1 — Linux test build fix (build-tagged Windows tests)
+- [x] P7.1 — Linux test build fix (build-tagged Windows tests)
 
 ### Phase 8 — Docs + version
 - [ ] P8.1 — `docs/DEVELOPMENT.md`