Add fertilizer to input-cost tools (regional $/ton, USDA AgTransport)

input_cost_trend/input_cost_series now accept six fertilizers (urea, uan,
anhydrous, dap, map, potash) alongside diesel, with an optional `geo` region
(default Cornbelt). Real $/ton + MoM/YoY change + seasonal context.

- client: pass geo through; add input_cost_geographies
- server: expand VALID_INPUTS; geo param + docstrings
- format already unit-aware ($/ton) and geo-aware
- README tools table now lists the reference/trend + input-cost tools
- CHANGELOG: regional fertilizer input-cost release notes
- tests: fertilizer $/ton + region formatting

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

CHANGELOG.md

@@ -3,6 +3,46 @@
 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 — Regional fertilizer input costs (real $/ton + change)
+
+Real U.S. **regional retail fertilizer prices** now feed the input-cost tools,
+so the advisor can reason about fertilizer the same way it does diesel and grain
+— actual dollars and the move, not an index. Source: **USDA AgTransport**
+(monthly, `$/ton`, history back to 2023).
+
+### Changed MCP tools (fertilizer added to the existing input-cost tools)
+
+- **`input_cost_trend(item, geo?, years=10)`** — `item` now accepts six
+  fertilizers in addition to `diesel`: **`urea`, `uan`, `anhydrous`** (anhydrous
+  ammonia), **`dap`, `map`, `potash`**. Returns the latest real `$/ton`,
+  month-over-month and year-over-year change ($ and %), and seasonal context.
+  - New optional **`geo`** = AgTransport region; **defaults to `Cornbelt`**.
+    Other regions: `U.S. Gulf NOLA`, `Northern Plains`, `Southern Plains`,
+    `Southeast`, `Northeast`, `California`, `Pacific Northwest`, `South Central`,
+    `Central Florida`, `Tampa` (not every product is published for every region).
+  - `diesel` is unchanged — national U.S. `$/gal`, weekly; `geo` is ignored for it.
+- **`input_cost_series(item, geo?)`** — raw monthly `$/ton` series for any
+  fertilizer (or the diesel `$/gal` weekly series), region-selectable.
+
+Seasonal percentile for fertilizer is computed over a short history (2023+), so
+it deepens over time; the price and YoY/MoM change are solid now.
+
+### API
+
+- `GET /api/data/input-cost-trend?item=&geo=&years=` — `geo` added.
+- `GET /api/data/input-cost-series?item=&geo=` — `geo` added.
+- `GET /api/data/input-cost-geographies?item=` — **new**: lists the regions a
+  given input has data for (plus its `default_geo`).
+
+### Example questions → tool calls
+
+| Ask | Call |
+|---|---|
+| Cornbelt urea price and how it's moved | `input_cost_trend(item="urea")` |
+| Anhydrous ammonia in the Southern Plains | `input_cost_trend(item="anhydrous", geo="Southern Plains")` |
+| Potash $/ton history | `input_cost_series(item="potash")` |
+| Which regions have DAP prices | (API) `GET /api/data/input-cost-geographies?item=dap` |
+
 ## 2026-05-30 — Grain price-received trends (real $ + change + seasonal)
 
 New national/seasonal reference layer (USDA NASS, corn back to 1908) — the