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/security/resources_linux.go

@@ -0,0 +1,18 @@
+// go:build linux
+// +build linux
+
+package security
+
+import "syscall"
+
+// checkVirtualMemoryLimit checks RLIMIT_AS (only available on Linux)
+func checkVirtualMemoryLimit(minVirtualMemoryMB uint64) error {
+	var vmLimit syscall.Rlimit
+	if err := syscall.Getrlimit(syscall.RLIMIT_AS, &vmLimit); err == nil {
+		if vmLimit.Cur != syscall.RLIM_INFINITY && vmLimit.Cur < minVirtualMemoryMB*1024*1024 {
+			return formatError("virtual memory limit too low: %s (minimum: %d MB)",
+				formatBytes(uint64(vmLimit.Cur)), minVirtualMemoryMB)
+		}
+	}
+	return nil
+}