new(opencost): Kubernetes cost monitoring (CNCF sandbox)

[tannevaled]

Summary

• Adds opencost.io recipe for OpenCost, the CNCF sandbox tool for real-time Kubernetes cost monitoring and allocation.
• Standard Go single-binary build from upstream ./cmd/costmodel package (no other cmd/ subdirs exist).
• Linker flags match upstream Makefile exactly (-X github.com/opencost/opencost/core/pkg/version.Version=..., GitCommit=...).
• Built and verified locally on darwin/arm64 with go 1.26.2; binary runs --help cleanly (cobra "Cost-Model metric exporter and data aggregator").

Test plan

• Local bk build opencost.io succeeds (darwin)
• Binary opencost --help prints cobra usage
• CI build on linux
• CI build on darwin