[FEATURE]: Trace sampling knob (OPENCODE_TRACE_RATIO) for OTel export

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

When OTEL_EXPORTER_OTLP_ENDPOINT is set, tracing is always-on: every span gets exported. A traced 80-minute session on a busy server produced ~320k spans / ~50MB to the collector, and today the only way to dial that down is removing the endpoint entirely.

I'd like an OPENCODE_TRACE_RATIO env var (default 1, i.e. unchanged behaviour) mapped to a ParentBased(TraceIdRatioBased) sampler. For example 0.1 would export ~10% of root traces while child spans stay consistent within a sampled trace. Patch ready to send.