fix: enable Windows cross-compilation

[hiroTamada]

Summary

• Move syscall.Stat_t usage to platform-specific files (stat_unix.go, stat_windows.go)
• On Windows, getFileOwnership() returns 0, 0 as UID/GID are Unix concepts

Context

The CLI's GoReleaser build was failing when cross-compiling for Windows:

build failed: # github.com/kernel/hypeman-go/lib
lib/cp.go:186:42: undefined: syscall.Stat_t
target=windows_amd64_v1

This fix allows the SDK to be cross-compiled for Windows targets.

Test plan

• go build ./... passes on Unix
• GOOS=windows GOARCH=amd64 go build ./... passes
• go test ./... passes

---

Note

Makes file ownership retrieval OS-specific to unblock Windows builds.

• Introduces getFileOwnership with stat_unix.go (uses syscall.Stat_t) and stat_windows.go (returns 0, 0)
• Updates cp.go to call getFileOwnership when Archive is set and removes direct syscall dependency
• Resolves Windows cross-compilation error due to syscall.Stat_t being undefined

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