l8k: Cluster Discovery

PREREQUISITE: Read ../k8s-launch-kit-shared/SKILL.md for install paths, global flags, and output modes.

Discover cluster hardware and produce a cluster-config.yaml describing NICs, GPUs, rails, and node groups.

Three things to know about the saved file:

• The resolved profile is persisted. Discovery fills missing profile fields from hardware/defaults, preserves values from --user-config, applies explicit CLI overrides last, and writes the final profile: block.
• North-south-only groups are dropped. A node group whose NICs are all north-south (BlueField DPUs / OOB NICs) produces no manifests and is omitted from cluster-config.yaml (it would otherwise consume an NV-IPAM subnet slice and shift the per-rail pools). North-south NICs alongside east-west NICs in a mixed group are still listed.
• Documentation comments are preserved. The field-reference comments from the source config (l8k-config.yaml or --user-config) are carried over into the saved cluster-config.yaml, with a Generated by l8k discover banner at the top.