[ci skip] update AGENTS.md + CLAUDE.md with SOPS workflow, add k8s-portal CI pipeline
AGENTS.md: added SOPS secrets management section, scripts/tg usage,
contributor onboarding steps, pull-through cache bypass notes.
CLAUDE.md: added SOPS workflow note, linux/amd64 build reminder,
versioned tag guidance for pull-through cache.
CI: new .woodpecker/k8s-portal.yml pipeline — auto-builds and deploys
the k8s portal when files under stacks/platform/modules/k8s-portal/files/
change on master push. Uses buildx for linux/amd64.