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/server.py

@@ -411,6 +411,27 @@ def input_cost_series(
         return fmt.fmt_price_series(client.input_cost_series(item=it, geo=g))
 
 
+@mcp.tool()
+def nutrient_cost(
+    geo: Annotated[
+        str | None,
+        Field(description="Fertilizer region (default 'Cornbelt'). Same regions as "
+                          "input_cost_trend (e.g. 'U.S. Gulf NOLA', 'Northern Plains')."),
+    ] = None,
+) -> str:
+    """Cheapest fertilizer per POUND of nutrient — "what's the best value for N (or P/K)?".
+
+    Converts each fertilizer's regional $/ton (USDA AgTransport, latest month) into
+    $/lb of N, P2O5, and K2O from its grade, ranks them, and names the cheapest
+    source of each nutrient (anhydrous usually wins on N; DAP/MAP are phosphate
+    buys; potash is the K source). Use THIS — not input_cost_trend ($/ton only) —
+    whenever the grower asks which fertilizer is the best buy / best nitrogen value.
+    UAN grade is assumed 32-0-0."""
+    g = geo.strip() if geo else None
+    with track("nutrient_cost", geo=g or ""):
+        return fmt.fmt_nutrient_cost(client.nutrient_cost(geo=g))
+
+
 @mcp.tool()
 def list_sources() -> str:
     """All active scrapers + their last-success timestamps and any pending failures."""