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Use Etc.nprocessors and handle sandboxed environments (#876)

## Summary

- Replace OS-specific `processor_count` implementation with Ruby's
`Etc.nprocessors`
- Rescue `Errno::EPERM`/`ENOENT` in `parent_command` when `ps` is
unavailable

## Motivation

The current `processor_count` method shells out to OS-specific commands
(`sysctl`, `/proc/cpuinfo`, `hwprefs`, etc.) to detect CPU count. This
was copied from the [parallel
gem](https://github.com/grosser/parallel/blob/v1.6.1/lib/parallel/processor_count.rb#L17-L51)
in 2015 when supporting older Ruby versions was necessary.

Ruby's `Etc.nprocessors` has been available since Ruby 2.2 (2014), and
overcommit requires Ruby 2.6+. It works cross-platform without shelling
out, which also fixes failures in sandboxed environments (e.g. macOS
Seatbelt) where `sysctl` calls are blocked at the syscall level —
causing `processor_count` to return 0 and breaking all hook validation.

Similarly, `parent_command` shells out to `ps` which can fail with
`Errno::EPERM` in sandboxed environments. The rescue returns `nil`,
which the caller (`amendment?`) already handles — hooks run against all
staged files instead of only amended ones.

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