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>

tests/test_format.py

@@ -75,6 +75,49 @@ def test_fmt_futures_no_quote():
     assert "No futures quote on file" in out
 
 
+def test_fmt_price_trend():
+    payload = {"commodity": "corn", "geo": "OH", "unit": "$/bu",
+               "source": "USDA NASS Quick Stats",
+               "trend": {"period": "2026-04-01", "value_cents": 468,
+                         "prev_cents": 459, "change_cents": 9, "change_pct": 2.0,
+                         "yoy_cents": 480, "yoy_change_cents": -12, "yoy_pct": -2.5,
+                         "seasonal": {"normal_cents": 435, "median_cents": 430,
+                                      "min_cents": 400, "max_cents": 480,
+                                      "percentile": 75, "vs_normal_pct": 7.6,
+                                      "sample_years": 4},
+                         "recent_direction": "up", "baseline_years": 10, "points": 120}}
+    out = fmt.fmt_price_trend(payload)
+    assert "corn — OH" in out
+    assert "$4.68/bu" in out
+    assert "+$0.09" in out and "+2.0%" in out      # MoM change
+    assert "−$0.12" in out and "−2.5%" in out      # YoY
+    assert "75th pct" in out
+
+
+def test_fmt_price_trend_empty():
+    out = fmt.fmt_price_trend({"commodity": "wheat", "geo": "US", "unit": "$/bu", "trend": None})
+    assert "No data on file" in out
+
+
+def test_fmt_price_series():
+    payload = {"commodity": "corn", "geo": "US", "unit": "$/bu", "count": 2,
+               "series": [{"period": "2026-03-01", "value_cents": 459},
+                          {"period": "2026-04-01", "value_cents": 468}]}
+    out = fmt.fmt_price_series(payload)
+    assert "Mar 2026" in out and "$4.59/bu" in out
+    assert "Apr 2026" in out and "$4.68/bu" in out
+
+
+def test_fmt_price_trend_diesel_units():
+    payload = {"commodity": "diesel", "geo": "US", "unit": "$/gal", "source": "EIA",
+               "trend": {"period": "2026-05-25", "value_cents": 552, "prev_cents": 560,
+                         "change_cents": -8, "change_pct": -1.4, "yoy_cents": None,
+                         "yoy_change_cents": None, "yoy_pct": None, "seasonal": None,
+                         "recent_direction": "down", "baseline_years": 10, "points": 200}}
+    out = fmt.fmt_price_trend(payload, label="retail diesel")
+    assert "$5.520/gal" in out   # 3-decimal $/gal formatting
+
+
 def test_fmt_inputs_lime_table():
     payload = {
         "product": "lime", "count": 2,