feat: nutrient_cost tool — cheapest fertilizer per lb of N/P/K

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>

tests/test_format.py

@@ -390,3 +390,44 @@ def test_fmt_summary_includes_best_for_today():
     assert "Mercer Landmark — St Henry" in out
     assert "$4.5800" in out  # corn last
     assert "No current-month local bid posted" in out  # soy fallback
+
+
+def test_fmt_nutrient_cost():
+    payload = {
+        "geo": "Cornbelt",
+        "source": "USDA AgTransport",
+        "products": [
+            {"item": "anhydrous", "label": "anhydrous ammonia", "unit": "$/ton",
+             "geo": "Cornbelt", "period": "2026-03-01", "price_cents_per_ton": 88062,
+             "analysis": {"grade": "82-0-0", "grade_assumed": False},
+             "cost_per_lb": {"n": 54, "p2o5": None, "k2o": None}},
+            {"item": "urea", "label": "urea", "unit": "$/ton", "geo": "Cornbelt",
+             "period": "2026-03-01", "price_cents_per_ton": 69812,
+             "analysis": {"grade": "46-0-0", "grade_assumed": False},
+             "cost_per_lb": {"n": 76, "p2o5": None, "k2o": None}},
+            {"item": "uan", "label": "UAN (28-32%)", "unit": "$/ton", "geo": "Cornbelt",
+             "period": "2026-03-01", "price_cents_per_ton": 47612,
+             "analysis": {"grade": "32-0-0", "grade_assumed": True},
+             "cost_per_lb": {"n": 74, "p2o5": None, "k2o": None}},
+            {"item": "potash", "label": "potash (0-0-60)", "unit": "$/ton",
+             "geo": "Cornbelt", "period": "2026-03-01", "price_cents_per_ton": 35938,
+             "analysis": {"grade": "0-0-60", "grade_assumed": False},
+             "cost_per_lb": {"n": None, "p2o5": None, "k2o": 30}},
+        ],
+        "cheapest": {"n": "anhydrous", "p2o5": None, "k2o": "potash"},
+    }
+    out = fmt.fmt_nutrient_cost(payload)
+    assert "Cornbelt" in out
+    assert "Cheapest Nitrogen (N):** anhydrous ammonia at $0.54/lb" in out
+    assert "Cheapest Potash (K₂O):** potash (0-0-60) at $0.30/lb" in out
+    # anhydrous (cheapest N) sorts above urea in the table
+    assert out.index("anhydrous ammonia |") < out.index("| urea |")
+    # UAN grade-assumed marker + footnote
+    assert "32-0-0*" in out
+    assert "UAN grade assumed 32-0-0" in out
+    # potash supplies no N -> em-dash in the N column
+    assert "—" in out
+
+
+def test_fmt_nutrient_cost_empty():
+    assert "No data on file" in fmt.fmt_nutrient_cost({"geo": "Cornbelt", "products": []})