chore: remove macOS CI builds — they miss real compilation errors

[miguelgila]

Problem

macOS CI builds provide false confidence. They miss real compilation errors because:

1. #[cfg(target_os = "linux")] gated code is skipped entirely — overlay, resize watcher, nsenter, and other Linux-only code is never compiled
2. Type resolution differs — controller feature types like OwnerReference and ObjectMeta resolve from kube::api on macOS but need k8s_openapi::apimachinery::... on Linux
3. Reaper only runs on Linux — it's a container runtime that uses Linux-specific syscalls (unshare, pivot_root, setns, etc.)

Evidence

• PR feat: ReaperDaemonJob CRD for node configuration tasks #58: OwnerReference not found error only surfaced in Linux CI, not macOS clippy
• This pattern has repeated multiple times with #[cfg(target_os = "linux")] gated code

Proposal

• Remove macOS build/test jobs from CI workflows
• Keep only Linux builds (native or cross-compilation)
• This saves CI minutes and eliminates false-green builds

Impact

• Faster CI (fewer jobs)
• No more "passes locally on macOS, fails in CI on Linux" surprises
• Developers on macOS should use cargo clippy --target x86_64-unknown-linux-gnu locally