test(ci): run agent integrations on Windows

[tiendungdev]

Summary

• make TestGlobalPointerBody_ShapeAndSentinel assert the OS-native path emitted by filepath.Join
• run the focused internal/agents test package in the existing native Windows CI job
• keep the broader Windows suite out of scope because it still pulls platform-specific parser/CGO test surfaces

Why

The Windows job currently compiles production packages but does not execute platform-specific tests. That leaves regressions such as the Windows atomic-write sharing-violation coverage in internal/agents/writer_windows_test.go dependent on local/on-demand verification.

Running go test ./internal/agents -count=1 on Windows exposed one portability false-failure before the Windows-only writer tests could be promoted to CI: TestGlobalPointerBody_ShapeAndSentinel hard-coded /active.md, while GlobalPointerBody intentionally emits the result of filepath.Join (\active.md on Windows).

This PR fixes that assertion and adds only the package-level test that now has a clean, bounded Windows responsibility.

Verification

• fail-first on Windows: go test ./internal/agents -count=1 failed at TestGlobalPointerBody_ShapeAndSentinel with the OS-native @\home\user\.gortex\instructions\active.md output
• gofmt -w internal/agents/instructions_test.go
• git diff --check
• the new windows-latest CI step is the authoritative post-fix Windows verification

Scope

No production behavior changes. No attempt to enable go test ./... on Windows.

[zzet]

@tiendungdev thank you for your contribution!