Narrations and diagnostics as Logs, split by severity #506

Both streams are OTel Logs; the split is SeverityNumber (narrations INFO/WARN/ERROR, diagnostics
DEBUG/TRACE), which is also the default-visibility split.

Carry log.iostream (stdout | stderr), the instrumentation scope as the subsystem classifier (the
logger name, e.g. devlore.provider.git), and code.function.name. trace_id / span_id are LogRecord
context fields auto-injected from the enclosing node span -- not attributes we set.

Epic: #449 · Feature: #504