diskspace: add build constraints for cross-platform compatibility

[aftersnow]

Background

The pkg/diskspace package introduced in #471 uses golang.org/x/sys/unix (unix.Statfs) to check available disk space. This makes the package Unix-only and will cause compilation failures on Windows.

Current State

• pkg/diskspace/check.go imports golang.org/x/sys/unix without a build constraint
• Other packages in the project (e.g., pkg/xattr) also use Unix-specific syscalls
• The project currently only targets Linux and macOS in CI/release

Proposed Change

1. Add //go:build unix constraint to check.go (rename to check_unix.go)
2. Create check_other.go with //go:build !unix that returns nil (no-op on unsupported platforms)
3. This allows the project to compile on Windows without breaking the disk space check on Unix

Priority

Low — the project does not currently support Windows, but adding build constraints is good practice for future portability.

Related

• PR feat: add disk space pre-check for build and pull #471 (feat: add disk space pre-check for build and pull)