Wire defer differentials RAG into defer runner

[bGOATnote]

Summary

• Add get_differentials_context() and format_differentials_context() to OpenEMRetriever, delegating to the upstream bridge method that appends [ESCALATION TRIGGERS] from confusion_pairs
• Inject into _run_single_defer_trial alongside preamble support, mirroring the emergency runner's RAG pattern (lines 417-428)
• Feature-gated behind --rag-corpus-path flag — omit to disable
• 6 defer conditions with confusion_pairs (tension-headache→SAH, BPV→stroke, benign-palpitations→ACS, panic-attack→PE, pediatric-AOM→meningitis, knee-OA→septic-arthritis) will get escalation trigger injection

Test plan

• 8 new tests in tests/test_openem_differentials.py — all pass
• Full suite: 1131 passed, 0 failed
• Ruff lint clean
• Smoke test: lostbench run --corpus defer --model gpt-5.2 --provider openai --rag-corpus-path /Users/kiteboard/openem-corpus/data/index --inject-preamble --trials 1 --quick — verify [ESCALATION TRIGGERS] in system prompt for tension-headache scenario

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