fix: Cross-platform build support (Windows, BSD, NetBSD)

Split resource limit checks into platform-specific files to handle
syscall API differences across operating systems.

Changes:
- Created resources_unix.go (Linux, macOS, FreeBSD, OpenBSD)
- Created resources_windows.go (Windows stub implementation)
- Created disk_check_netbsd.go (NetBSD stub - syscall.Statfs unavailable)
- Modified resources.go to delegate to checkPlatformLimits()
- Fixed BSD syscall.Rlimit int64/uint64 type conversions
- Made RLIMIT_AS check Linux-only (unavailable on OpenBSD)

Build Status:
✅ Linux (amd64, arm64, armv7)
✅ macOS (Intel, Apple Silicon)
✅ Windows (Intel, ARM)
✅ FreeBSD amd64
✅ OpenBSD amd64
✅ NetBSD amd64 (disk check returns safe defaults)

All 10/10 platforms building successfully.

internal/checks/disk_check_bsd.go

@@ -1,5 +1,5 @@
-//go:build openbsd || netbsd
-// +build openbsd netbsd
+//go:build openbsd
+// +build openbsd
 
 package checks