Fix Dup2 usage

[canercidam]

This fixes the linux/arm64 build.

[claude]

Claude finished @canercidam's task in 50s —— View job

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PR Review

Scope: Replaces syscall.Dup2 with unix.Dup2 in utils/detached.go to fix the linux/arm64 build, and promotes golang.org/x/sys from indirect to direct in go.mod.

Assessment: The fix is correct and minimal.

• syscall.Dup2 is not defined on linux/arm64 (and riscv64, loong64) because the kernel does not expose the dup2 syscall there — only dup3. golang.org/x/sys/unix.Dup2 papers over this by calling Dup3(oldfd, newfd, 0) on those architectures, so the behavior is equivalent on the architectures that already worked, and it now compiles on arm64.
• Moving golang.org/x/sys from indirect to direct is the right hygiene since it's now imported directly.
• One minor semantic difference worth being aware of (not a blocker here): on arm64-style builds, unix.Dup2 dispatches through Dup3, which returns EINVAL when oldfd == newfd (whereas dup2 is a no-op in that case). The three call sites here dup a freshly-opened fd onto 0/1/2, so they can't collide — no action needed, just flagging.
• No build tag on utils/detached.go, but the file already uses SysProcAttr{Setsid: true} etc., so it was implicitly Unix-only before this PR and remains so.

Nothing that needs attention.
• caner/fix-build-syscall-dup