When OTEL_EXPORTER_OTLP_ENDPOINT (or _TRACES_ENDPOINT) is set, register a
global OTLP/proto TracerProvider and switch on the agent loop's telemetry
so its invoke_agent / chat / execute_tool spans reach a collector instead
of a no-op tracer. Honors the standard OTEL_* env contract
(OTEL_SERVICE_NAME, OTEL_SDK_DISABLED, OTEL_TRACES_EXPORTER=none) and the
OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT capture toggle; no-op
when no endpoint is configured.
Uses the exporter-trace-otlp-proto 2.x line, which exports cleanly under
Bun (the 1.x line deadlocks). Adds a flushTelemetryExport() helper and a
periodic + signal-driven flush so long-lived processes surface spans.
This makes the existing telemetry usable from hosts that spawn omp as a
child process (e.g. an ACP harness), where an in-process provider
registered by the parent can't reach the child process.
