epa_ppls: narrow row-crop filter to corn/soy/wheat only

App focus is corn, soybeans, and wheat. Dropping the broader
US-row-crops allowlist (cotton/rice/sorghum/milo/barley/oats/rye/
sunflower/peanut/sugar-beet/dry-bean/canola/alfalfa).

Empirical impact (random N=100 sample): broad list matched 17/100
products, narrow list matches 16/100 — only 6% reduction, because
corn/soy/wheat dominate ag-chem registrations so thoroughly that
products registered for cotton/sorghum/etc. are almost always
co-registered for one of corn/soy/wheat. One sampled product was
dropped: a peanut-only herbicide (2749-614).

Verified live: 524-475 Roundup + 524-591 Warrant kept (CORN/SOYBEAN
sites); 2749-614 AG36448 (PEANUTS only) correctly filtered.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

sources.json

@@ -16,6 +16,6 @@
     "scraper": "scrape.sources.epa_ppls",
     "scraper_version": "0.1.0",
     "license_note": "US federal government — public domain (no ToS restriction)",
-    "scope_filter": "row-crop only — products with at least one site matching CORN, MAIZE, POPCORN, SOYBEAN(S), COTTON, WHEAT, RICE, SORGHUM, MILO, BARLEY, OATS, RYE, SUNFLOWER(S), PEANUT(S), SUGAR BEET(S), DRY/FIELD BEAN(S), CANOLA, RAPESEED, or ALFALFA (word-boundary match). Pass --no-row-crop-filter to scrape the full PPLS universe."
+    "scope_filter": "corn / soybean / wheat only — products with at least one site matching CORN, MAIZE, POPCORN, SOYBEAN(S), or WHEAT (word-boundary match). Hits ~16% of the PPLS universe in sampling. Pass --no-row-crop-filter to scrape the full PPLS universe."
   }
 ]