Add input_cost_trend / input_cost_series MCP tools (EIA diesel)

Real input price + WoW/YoY change + seasonal for diesel ($/gal). Formatters now
handle the item/label payload shape. Changelog updated.

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

CHANGELOG.md

@@ -26,8 +26,21 @@ not an index.
 - `GET /api/data/price-series?commodity=&geo=&start_year=&end_year=` — raw series.
 - `GET /api/data/price-geographies?commodity=` — which geos (US + states) exist.
 
-All grain: corn/soy/wheat, all 50 states + US. (Input-cost trends — diesel,
-fertilizer — coming next.)
+All grain: corn/soy/wheat, all 50 states + US.
+
+### Input-cost tools (real $ + change)
+
+- **`input_cost_trend(item, years=10)`** — real input price with the move.
+  Currently `item="diesel"` (EIA U.S. retail $/gal, weekly, back to 1994):
+  latest price + week-over-week and year-over-year change + seasonal
+  percentile/range. (For current fertilizer $/ton, `current_input_price` (DTN)
+  still applies; more inputs extend this same tool.)
+- **`input_cost_series(item)`** — raw historical series for an input.
+- API: `GET /api/data/input-cost-trend?item=&years=`,
+  `GET /api/data/input-cost-series?item=`.
+
+USDA stopped publishing dollar input prices in 2014, so these use real-dollar
+sources (EIA) rather than an index.
 
 ## 2026-05-30 — Source geo + many more locations