#2883 Docs: document serve and worker trace structure in the OpenTelemetry reference

The manual's content/manual/reference/opentelemetry.md has no section on how swamp serve and swamp worker traces are structured, and its CLI Root Span section implies every span sits under swamp.cli. swamp-club#2467 (PR swamp-club/swamp#2763) changed this.

Suggested content for a new 'Traces in swamp serve and workers' section:

• Root spans for background work. HTTP requests are SERVER root spans. Each poll cycle is a root swamp.serve.poll span. Its swamp.serve.poller attribute is config, access_data, runtime_data or grants_directory, and its sync-gate, datastore sync and datastore extension spans nest under it. Each scheduled fire is a root swamp.scheduled.fire span with the workflow run under it. A WebSocket or queued webhook run without traceparent starts its own trace. Other loops (heartbeats, GC, audit flushes, token revalidation) emit no per-tick spans.
• Worker dispatches. A dispatch joins the run's trace when serve sends trace headers: swamp.remote.dispatch sits under the step's swamp.model.method, then the runner's swamp.cli. swamp.serve.data_plane spans, with a swamp.data_plane.operation attribute, record the runner's data-plane reads and writes in the run's trace.
• Runner environment. Runners never inherit TRACEPARENT from the worker's environment. Serve no longer ships its OTEL_* settings or trace context to workers, so runners export with the worker host's own OTel settings. Configure OTEL_* on each worker host.
• Console exporter. Console span output from a runner goes to stderr and appears in the worker log at debug level.
• Export at exit. A process waits up to the 10s exporter timeout for in-flight span exports when it exits, including on the error path.
• Correction. The Coverage section's statement that worker subprocesses receive trace context via TRACEPARENT is still true, but only from the dispatch's own trace headers.