feat: add getCPU() with cgroup limit support

[loks0n]

Summary

• Adds System::getCPU(): float, returning CPU capacity that honours cgroup v2 (cpu.max) and cgroup v1 (cpu.cfs_quota_us / cpu.cfs_period_us) — so Docker/Kubernetes limits like --cpus=0.5 or limits.cpu: 500m are reflected as 0.5.
• Also honours cpuset pinning (--cpuset-cpus=0-3); when both a cfs quota and a cpuset are configured, the more restrictive value is returned.
• Falls back to the host's physical core count when no cgroup limit is set, and uses the existing sysctl / wmic paths on Darwin / Windows.
• Marks getCPUCores() as @deprecated in favour of the new method.

Test plan

• php -l clean on src/System/System.php and tests/System/SystemTest.php
• Added testGetCPU asserting float and > 0
• Run full PHPUnit suite in CI
• Manual verification inside a container with --cpus=0.5 returns 0.5
• Manual verification with --cpuset-cpus=0-1 returns 2.0

[greptile-apps]

Greptile Summary

• Introduces getCPU(): float that correctly honours cgroup v2 (cpu.max) and cgroup v1 (cpu.cfs_quota_us/cpu.cfs_period_us) quotas, cpuset pinning, and falls back to /proc/cpuinfo on Linux; Darwin and Windows use sysctl/wmic with proper error handling.
• All five issues raised in the prior review cycle have been addressed: Windows multi-socket sum uses preg_match_all + array_sum, unreadable /proc/cpuinfo now throws, Darwin/Windows both throw on failure, and the cgroup v2 cpuset.cpus.effective always-present false-positive is neutralised by comparing against /sys/devices/system/cpu/online.
• getCPUCores() is marked @deprecated in favour of the new method.

Confidence Score: 5/5

This PR is safe to merge; no new P1 or P0 issues were found and all previously flagged concerns have been resolved.

Every issue from the prior review thread has a concrete fix in the diff. The cgroup parsing logic handles edge cases (max keyword, zero-period guard, unrestricted cpuset detection). No new bugs were identified.

No files require special attention.

Important Files Changed

Filename	Overview
src/System/System.php	Adds getCPU(): float with cgroup v2/v1 quota support, cpuset detection (with unrestricted guard), private helper methods, and deprecates getCPUCores(). All five issues from the prior review thread have been addressed.
tests/System/SystemTest.php	Adds testGetCPU() asserting the return is a float greater than 0, consistent with the existing testGetCPUCores() pattern.

_{Reviews (5): Last reviewed commit: "fix: guard against inverted ranges in co..." | Re-trigger Greptile}