Add code review guidelines for tier markers and hardware checks

[rnetser]

Summary

This PR adds code review guidelines to improve review consistency and prevent common issues:

• Tier marker guidance in CLAUDE.md: Added reviewer instruction to check for missing tier markers (@pytest.mark.tier1/tier2/tier3) on new tests, referencing pytest.ini for available markers
• Hardware fixture checks in CodeRabbit workflow: Added guidance to detect hardware-specific fixtures (gpu_nodes, sriov_node, sriov_namespace, etc.) and ensure proper skip conditions are in place to prevent collection errors on clusters without specialized hardware

Context

These guidelines were informed by discussions and issues from:

• PR add dedicated cpu instancetype test RedHatQE/openshift-virtualization-tests#3697 discussion on tier marker review scope
• PR net, sriov: Use existing sriov policy RedHatQE/openshift-virtualization-tests#3072 which introduced hardware-related collection failures
• PR net, sriov: Refactor sriov_node_policy RedHatQE/openshift-virtualization-tests#3347 which addressed similar hardware fixture skip condition patterns

Why

• Tier markers: Ensures all new tests are properly categorized for test execution filtering, preventing unmarked tests from being missed in CI runs
• Hardware checks: Prevents collection errors when tests use hardware-specific fixtures without proper skip conditions, which causes test suite failures on clusters lacking specialized hardware (GPU, SR-IOV, etc.)