Add price_trend / price_series MCP tools (USDA NASS grain)

Real $/bu price + MoM/YoY change + seasonal percentile context for corn/soy/
wheat, US + states, deep history. Wraps the new /api/data/price-trend and
/api/data/price-series endpoints. Changelog updated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ag_bids_mcp/client.py

@@ -70,6 +70,16 @@ def best(commodity: str) -> dict:
     return _get("/api/data/best", commodity=commodity)
 
 
+def price_trend(commodity: str, geo: str = "US", years: int = 10) -> dict:
+    return _get("/api/data/price-trend", commodity=commodity, geo=geo, years=years)
+
+
+def price_series(commodity: str, geo: str = "US",
+                 start_year: int | None = None, end_year: int | None = None) -> dict:
+    return _get("/api/data/price-series", commodity=commodity, geo=geo,
+                start_year=start_year, end_year=end_year)
+
+
 def futures(commodity: str, delivery: str | None = None) -> dict:
     return _get("/api/data/futures", commodity=commodity, delivery=delivery)