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>

CHANGELOG.md

@@ -3,6 +3,32 @@
 Notes for clients/agents that consume the ag-bids MCP tools and the underlying
 `ag-monitor` `/api/data/*` HTTP API. Newest first.
 
+## 2026-05-30 — Grain price-received trends (real $ + change + seasonal)
+
+New national/seasonal reference layer (USDA NASS, corn back to 1908) — the
+macro benchmark to compare local cash bids against. Real prices and the change,
+not an index.
+
+### New MCP tools
+
+- **`price_trend(commodity, geo="US", years=10)`** — monthly price *received by
+  farmers* ($/bu) with the move: latest real price, **month-over-month and
+  year-over-year change ($ and %)**, and seasonal context (percentile vs the
+  same month over the last N years, normal, and range). `geo` is `US` or any
+  2-letter state. Conclusions, not rows.
+- **`price_series(commodity, geo="US", start_year?, end_year?)`** — raw monthly
+  series ($/bu) for charting/drill-down (defaults to the recent window).
+
+### API
+
+- `GET /api/data/price-trend?commodity=&geo=&years=` — computed trend (cents +
+  `change_cents`/`change_pct`/`yoy_*` + `seasonal`).
+- `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.)
+
 ## 2026-05-30 — Source geo + many more locations
 
 ### Per-source geo (API + MCP)