add-middleware

# Add a Middleware Type ## Companion Guidance Use `karpathy-guidelines` alongside this skill for implementation or review work. Keep changes scoped, surface assumptions, and define focused validation before editing. NeMo Relay supports guardrails (validate/gate) and intercepts (transform) at various pipeline stages. Adding a new middleware type requires changes across all layers. Use this skill when introducing a new middleware registration surface or adding middleware behavior to a new pipeline stage. ## Lock The Design First Decide these before editing code: - Is this for tools, LLMs, marks, scope events, or a combination? - Is it a conditional guardrail, sanitize guardrail, request intercept, or execution intercept? - Does it run on request input, inner callable execution, stream chunks, or final response output? - Is the callback fallible, and how should callback failures propagate? - Does it need both global and scope-local registration? - What should subscribers and exporters observe in the event payload after this middleware runs? - If this is an event sanitizer, which of `data`, `category_profile`, and `metadata` can change, and is the event used only as immutable context? ## Pipeline Order Refer to `docs/about-nemo-relay/concepts/middleware.mdx` for the full diagrams. - **Tool execute**: conditional guardrails -> request intercepts -> sanitize request (for events) | execution intercept chain(callable) -> sanitize response - **LLM execute**: co...