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.

Author: matthewbjones

lib/overcommit/utils.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require 'etc'
 require 'pathname'
 require 'overcommit/os'
 require 'overcommit/subprocess'
@@ -150,6 +151,9 @@ def parent_command
         else
           `ps -ocommand= -p #{Process.ppid}`.chomp
         end
+      rescue Errno::EPERM, Errno::ENOENT
+        # Process information may not be available, such as inside sandboxed environments
+        nil
       end
 
       # Execute a command in a subprocess, capturing exit status and output from
@@ -212,40 +216,8 @@ def execute_in_background(args)
       end
 
       # Return the number of processors used by the OS for process scheduling.
-      #
-      # @see https://github.com/grosser/parallel/blob/v1.6.1/lib/parallel/processor_count.rb#L17-L51
-      def processor_count # rubocop:disable all
-        @processor_count ||=
-          begin
-            if Overcommit::OS.windows?
-              require 'win32ole'
-              result = WIN32OLE.connect('winmgmts://').ExecQuery(
-                'select NumberOfLogicalProcessors from Win32_Processor'
-              )
-              result.to_enum.collect(&:NumberOfLogicalProcessors).reduce(:+)
-            elsif File.readable?('/proc/cpuinfo')
-              IO.read('/proc/cpuinfo').scan(/^processor/).size
-            elsif File.executable?('/usr/bin/hwprefs')
-              IO.popen('/usr/bin/hwprefs thread_count').read.to_i
-            elsif File.executable?('/usr/sbin/psrinfo')
-              IO.popen('/usr/sbin/psrinfo').read.scan(/^.*on-*line/).size
-            elsif File.executable?('/usr/sbin/ioscan')
-              IO.popen('/usr/sbin/ioscan -kC processor') do |out|
-                out.read.scan(/^.*processor/).size
-              end
-            elsif File.executable?('/usr/sbin/pmcycles')
-              IO.popen('/usr/sbin/pmcycles -m').read.count("\n")
-            elsif File.executable?('/usr/sbin/lsdev')
-              IO.popen('/usr/sbin/lsdev -Cc processor -S 1').read.count("\n")
-            elsif File.executable?('/usr/sbin/sysctl')
-              IO.popen('/usr/sbin/sysctl -n hw.ncpu').read.to_i
-            elsif File.executable?('/sbin/sysctl')
-              IO.popen('/sbin/sysctl -n hw.ncpu').read.to_i
-            else
-              # Unknown platform; assume 1 processor
-              1
-            end
-          end
+      def processor_count
+        @processor_count ||= Etc.nprocessors
       end
 
       # Calls a block of code with a modified set of environment variables,