maintain-observability

# Maintain Observability Surfaces ## Companion Guidance Use `karpathy-guidelines` alongside this skill for implementation or review work. Keep changes scoped, surface assumptions, and define focused validation before editing. Use this skill when changing event fields, exporter behavior, subscriber config, or binding parity for ATIF or the `full`, `gen_ai`, and `openinference` OpenTelemetry projections. ## Surfaces To Keep In Sync - Core event model and emitted fields - `crates/core/src/observability/atif.rs` - `crates/core/src/observability/otel.rs` - `crates/core/src/observability/openinference.rs` - FFI and binding-native wrappers where the config or lifecycle is exposed - Python, Go, and Node.js config objects and subscriber/exporter methods - Observability config version 3, where one `opentelemetry` section contains typed endpoints and OpenInference has no standalone public surface - Docs under `docs/about-nemo-relay/concepts/subscribers.mdx` and `docs/configure-plugins/observability/` ## Design Checklist - [ ] Is this an event-model change, exporter-config change, or lifecycle change? - [ ] Do all bindings expose the same logical knobs and semantics? - [ ] Does every OpenTelemetry endpoint require a type and nonblank destination? - [ ] Does each endpoint resolve `header_env` values at activation and reject missing, blank, or duplicate headers? - [ ] Do layered ATOF sink, ATIF storage, and OpenTelemetry endpoint lists concatenate with higher-precedence entr...