docs: add DEBUGGING-TRACING.md (operational tracing/log debugging recipes)
Adds .claude/rules/DEBUGGING-TRACING.md — operational recipes for debugging the dataprep/RAG pipeline with VictoriaTraces + ArangoDB ingestion_log (Jaeger API gotchas, dataprep span taxonomy, AQL label-quality queries, source/chunk locations, the base64 remote-script pattern, and the gotchas that cost real debugging time). Generic, not deployment-specific. Linked from CLAUDE.md alongside OBSERVABILITY.md.