feat(otel): enable OTLP tracing through standard environment configuration

First part of #31 .

Services can now enable OTLP tracing through OTEL_EXPORTER_OTLP_ENDPOINT or OTEL_EXPORTER_OTLP_TRACES_ENDPOINT without configuring an endpoint in code.

The SDK resolves environment configuration independently for each signal, while explicit builder endpoints and protocols take precedence. HTTP export supports TLS, and gRPC export preserves native-root handling. A trace-specific endpoint alone does not enable metrics.

This MR covers exporter initialization and environment configuration. Sampling defaults and span helpers are left for follow-up MRs.