latest_prices: add include_expired passthrough for historical data

ag-monitor now drops expired delivery months from /api/data/latest by
default (current + future only). Expose the opt-in through the MCP so the
advisor can fetch past months when a farmer explicitly asks for history,
while the default stays current + future.

- client.latest(include_expired=False): forwards include_expired=true only
  when set (None is dropped), so the default query string is unchanged.
- latest_prices tool: new include_expired arg with a description steering
  the model to set it only on an explicit historical request.

Tests: client passthrough (default omits the param; opt-in forwards it).
43 passed.

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

README.md

@@ -83,7 +83,7 @@ auto-updates it every 5 minutes.
 | `price_series(commodity, geo?, start_year?, end_year?)` | Raw monthly price-received series for charting |
 | `input_cost_trend(item, geo?, years?)` | Real input cost + change. `item` = `diesel` (U.S. `$/gal`, weekly, EIA) or a fertilizer `$/ton` (`urea`/`uan`/`anhydrous`/`dap`/`map`/`potash`, monthly, USDA AgTransport); `geo` selects the fertilizer region (default `Cornbelt`) |
 | `input_cost_series(item, geo?)` | Raw historical series for an input cost (diesel `$/gal` or fertilizer `$/ton`, region-selectable) |
-| `latest_prices(commodity?, source?, delivery?, kind?, zip?, lat?, lng?, radius_miles?, include_expired?)` | Live snapshot table; every filter optional (`kind` = `grain`/`fertilizer`). Current + future delivery months only by default; pass `include_expired=true` for past months (use only on an explicit historical request). Location filter (zip **or** lat/lng + `radius_miles`) keeps nearby sources, nearest-first, with distance |
+| `latest_prices(commodity?, source?, delivery?, kind?, zip?, lat?, lng?, radius_miles?)` | Live snapshot table; every filter optional (`kind` = `grain`/`fertilizer`). Location filter (zip **or** lat/lng + `radius_miles`) keeps nearby sources, nearest-first, with distance |
 | `price_history(commodity?, source?, delivery?, days?)` | Time series per (elevator, crop, delivery); **every filter optional** — omit `commodity` to span all crops. Shows bid + basis trend + futures |
 | `basis_movement(commodity?, source?, delivery?, days?)` | Aggregated basis trend, one headline line per crop (the cheap overview) |
 | `basis_detail(commodity?, source?, delivery?, days?)` | Per-(elevator, crop, delivery) basis first→last drill-down |