Create an ARC-1 extension

Guides a developer through building an ARC-1 extension — a local plugin that adds Custom_* tools to an ARC-1 instance without forking, reusing ARC-1's authenticated SAP client, the 7-scope + allow* safety ceiling, audit, and PP. Encodes the learnings from building the framework (PR1–PR5) and verifying it live on S/4HANA.

Ground truth — read these first, mirror them:

• User guide (point the developer here): docs_page/extensions.md — the canonical how-to (tiers, ctx.http/ctx.run, security, CF/Docker deployment). Published at the docs site under Using ARC-1 → Extensions (Custom Tools).
• Spec: docs/research/2026-06-17-extension-framework-spec.md (v1) + extension-framework-v2-spec.md (what's deferred).
• Worked sample: arc-mcp/arc-1-extension-sample — ADT + OData reads, a manifest tool, Custom_RunClass (gated execute), an OData write (Custom_CreateSalesOrder), and a full LISA custom-ICF integration (Custom_ListLanguages/GetTranslation/SetTranslation) — all live-verified on S/4HANA (real HTTP 201/200 writes). Copy the closest tool and adapt.