fix(reranker): dampen skew in adaptive relevance formula

Promotes MR !204 (merged) (Roman Chestnov) — validated formula tuning from el-salvador, currently only on el-salvador-contextual. Two-line change in adaptive_context_selection: skew becomes a tanh-dampened multiplier on avg_score (numerator) and is dropped from the confusion-cost denominator. Verified: file is byte-identical to el-salvador-contextual's state; test_reranker.py unchanged (behavioral assertions only); ruff check genie-ai-overlay/ passes.