feat: nutrient_cost tool — cheapest fertilizer per lb of N/P/K (#1)

Co-authored-by: claude <claude@jpaul.io>
Co-committed-by: claude <claude@jpaul.io>

ag_bids_mcp/format.py

@@ -22,6 +22,12 @@ def _ton(cents: Optional[int]) -> str:
     return f"${cents / 100:.2f}"
 
 
+def _per_lb(cents: Optional[int]) -> str:
+    if cents is None:
+        return "—"
+    return f"${cents / 100:.2f}"
+
+
 def _basis(cents: Optional[int]) -> str:
     if cents is None:
         return "—"
@@ -207,6 +213,57 @@ def fmt_price_series(payload: dict, max_points: int = 60) -> str:
     return "\n".join(head + body) + "\n"
 
 
+_NUTRIENT_LABEL = {"n": "Nitrogen (N)", "p2o5": "Phosphate (P₂O₅)", "k2o": "Potash (K₂O)"}
+
+
+def fmt_nutrient_cost(payload: dict) -> str:
+    """Cheapest fertilizer per pound of N / P2O5 / K2O for a region."""
+    geo = payload.get("geo") or "Cornbelt"
+    src = payload.get("source") or "USDA AgTransport"
+    products = payload.get("products") or []
+    cheapest = payload.get("cheapest") or {}
+    if not products:
+        return f"### Fertilizer value per nutrient — {geo}\n\nNo data on file.\n"
+    by_item = {p["item"]: p for p in products}
+
+    lines = [f"### Fertilizer value per pound of nutrient — {geo}", ""]
+    for nut in ("n", "p2o5", "k2o"):
+        it = cheapest.get(nut)
+        prod = by_item.get(it or "")
+        if prod is not None:
+            c = (prod.get("cost_per_lb") or {}).get(nut)
+            lines.append(
+                f"- **Cheapest {_NUTRIENT_LABEL[nut]}:** "
+                f"{prod.get('label') or it} at {_per_lb(c)}/lb"
+            )
+    lines += ["", "| Product | Grade | $/ton | $/lb N | $/lb P₂O₅ | $/lb K₂O |",
+              "|---|---|---:|---:|---:|---:|"]
+
+    def _nkey(p: dict):
+        v = (p.get("cost_per_lb") or {}).get("n")
+        return (v is None, v if v is not None else 0)
+
+    for p in sorted(products, key=_nkey):  # cheapest N first, blanks last
+        a = p.get("analysis") or {}
+        cpl = p.get("cost_per_lb") or {}
+        grade = (a.get("grade") or "") + ("*" if a.get("grade_assumed") else "")
+        lines.append(
+            f"| {p.get('label') or p['item']} | {grade} | {_ton(p.get('price_cents_per_ton'))} | "
+            f"{_per_lb(cpl.get('n'))} | {_per_lb(cpl.get('p2o5'))} | {_per_lb(cpl.get('k2o'))} |"
+        )
+
+    period = next((p.get("period") for p in products if p.get("period")), None)
+    assumed = any((p.get("analysis") or {}).get("grade_assumed") for p in products)
+    foot = f"_Source: {src}"
+    if period:
+        foot += f" · as of {_ym(period)}"
+    foot += " · $/lb = $/ton ÷ (analysis% × 2000 lb)"
+    if assumed:
+        foot += " · *UAN grade assumed 32-0-0"
+    lines.append("\n" + foot + "_")
+    return "\n".join(lines) + "\n"
+
+
 # ---------- futures quote + change ----------