Docs and CHANGELOG reference telemetry env vars/flags the code doesn't read

Severity: medium (documentation).

### Summary

Operator docs (`docs/src/operator/monitoring.md:5-7,78-84`, `docs/external/src/operators.md:55-60`) document `OTEL_ENABLED` / `OTEL_TRACES_ENDPOINT`, but the code only reads `OTEL_EXPORTER_OTLP_TRACES_ENDPOINT` / `OTEL_EXPORTER_OTLP_ENDPOINT` (`crates/node/src/logging.rs:41-50`). The CHANGELOG (v0.4.1) additionally claims `--enable-otel`/`--otel-endpoint` CLI flags that do not exist (`bin/node/src/main.rs:11-39`).

### Impact

An operator following the published docs gets zero telemetry, silently — compounded by the exporter build error being swallowed at `logging.rs:96` (a malformed endpoint disables tracing with no log).

### Recommendation

Fix both doc trees and the changelog to the actual `OTEL_EXPORTER_OTLP_*` variables; log a warning (or fail startup) when an explicitly-requested OTEL exporter fails to build.

---
Part of #114.