Wire tracing subscriber: fmt + OTLP layers

[shaneutt]

Compose tracing subscriber with both fmt (console/file logging) and OTLP (trace export) layers sharing context.

Requirements

• Single tracing subscriber combining fmt and OTLP layers
• Shared span context between logging and tracing
• Trace ID available in log output (fmt layer)
• Configurable: OTLP layer opt-in, fmt layer always present
• Graceful initialization: OTLP layer skipped if not configured

Implementation Notes

• Use tracing-subscriber Registry with layered approach
• tracing-opentelemetry for OTLP layer
• Replace existing fmt-only subscriber setup

Parent epic: #10