Sync from docs: add Kubernetes/Helm Charts self-hosting guidance (powersync-docs #452)

[bean1352]

Generated by Claude. Review carefully before merging.\n\nSource docs PR: https://github.com/powersync-ja/powersync-docs/pull/452\n\n**What changed in docs:** Added a new deployment platform page for AWS EKS, documenting how to run the self-hosted PowerSync Service on Kubernetes using the community Helm charts (powersync-community/powersync-helm-chart). Covers workload sizing, observability, ingress requirements, horizontal scaling, and the multi-instance pattern.\n\nSkill updates in this PR:\n- skills/powersync/references/powersync-service.md — added a new ### Kubernetes / Helm Charts subsection under ## Service Configuration (Self-hosted). Content covers:\n - Helm chart repo link (source of truth for values and upgrade notes)\n - Replication warm-standby constraint (only 1 pod replicates at a time — do not scale replicas horizontally)\n - API connection cap (target 100/pod, hard cap 200 → PSYNC_S2304 errors)\n - NODE_OPTIONS note (leave --max-old-space-size-percentage=80 as-is)\n - Ingress requirements (proxy-buffering: \"off\" required for streaming, HTTP/2 + WebSocket, dedicated subdomain)\n - Multi-instance scaling pattern and the client-pinning requirement (switching instances triggers full resync)\n - Prometheus metrics port (9464) and key signals\n- Updated frontmatter description and tags to include kubernetes, helm, eks\n\nNotes for reviewer:\n- The docs page recommends a NetworkPolicy (networkPolicy.enabled: true) in production — included as a note in the Observability section.\n- The AGENTS.md "Setup Paths" section currently lists Docker as the only self-hosted path. This PR does not add a Kubernetes path there, since the docs change is a deployment guide rather than an onboarding flow change, and existing routing in SKILL.md already directs self-hosting questions to powersync-service.md. A reviewer may want to add a brief mention of Helm as a self-hosted alternative in AGENTS.md Path 3/4 — flagged here but not done to keep the diff minimal.\n- No new reference file was created; the Kubernetes content fits naturally within the existing powersync-service.md scope.

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