New agbids__nutrient_cost(geo) tool: wraps ag-monitor's /api/data/nutrient-cost
and formats a per-nutrient value comparison for the advisor — cheapest source of
N / P2O5 / K2O + a ranked $/lb table. This is what answers "which fertilizer is
the best nitrogen value per dollar?" (input_cost_trend is $/ton only). UAN grade
shown as assumed 32-0-0.
client.nutrient_cost + fmt.fmt_nutrient_cost (+ _per_lb). 42 tests pass.
For planning #90.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>