Two new trial sources: LG Seeds + AgriGold plot reports (+2,307 cross-vendor yield trials)

Adds the **first non-Syngenta trial coverage** to the corpus:

| Source | Docs | Publisher | URL pattern |
|---|---|---|---|
| lg_plot_reports | 1,304 | LG Seeds (AgReliant) | lgseeds.com/performance/{crop} JSON XHR |
| agrigold_plot_reports | 1,003 | AgriGold (AgReliant) | agrigold.com/{crop}/performance/{crop}-yield-results |

Total trial coverage now: gh_plot_reports (4,299) + agripro_trials (14) +
lg_plot_reports (1,304) + agrigold_plot_reports (1,003) = 6,620 trial docs.

**Both scrapers follow the gh_plot_reports template** — same RateLimitedSession
primitive, same TrialResult/PlotReport dataclass shape, same data_type="trial"
sidecar convention. The trial chunker (`rag/chunk.py:_render_trial_chunk`) is
extended to recognize both new sources; they share `_render_gh_plot_chunk`
since their sidecars are structurally identical (just different brand label).

**LG specifics:**
- POST `/performance/{crop}/GetPlots/` returns sparse listing (id, year, lat/lng)
- GET `/performance/{crop}/GetPlotData/?PlotId=X&IsSilage=Y` returns full detail
  with state, cooperator, planting/harvest dates, and **top-5 hybrids** (LG +
  competitors). Top-5 is what LG publishes publicly; not the full ranking.
- 4 crops: corn (963), soybeans (287), sorghum (10), silage (50) — first
  alfalfa absent because LG doesn't run alfalfa plots; that's variety-only data.
- 301 gotcha: www.lgseeds.com redirects to lgseeds.com which drops POST body,
  so the scraper hits the apex host directly.

**AgriGold specifics:**
- Listing: GET `/{crop}/performance/{crop}-yield-results?harvestYear={year}`
  (server-rendered HTML, ~1MB; 408 corn plots in 2025 alone)
- Detail: GET `/{crop_url}/performance/{slug}/{plot_id}` returns the **full
  ranking** (not just top-5) plus rich plot management metadata: tillage,
  previous crop, fungicide, herbicide, insecticide, irrigation, soil type,
  row width, population. Most metadata-rich of the three trial sources.
- Soybean URL slug is singular: `/soybeans/performance/soybean-yield-results/`
- Columns: Rank | Brand | Product | Trait | Ck | H20 (moisture) | Test Wt. |
  Yield | Adj Yield (check-adjusted)
- 2 crops: corn (849) + soybeans (157)

**Indexer needs no changes** — `rag/index.py` auto-discovers any directory
under corpus/ and routes by data_type. Both new sources flow into the
existing trial collection and surface via `search_trials`.

Years scraped: 2024+2025 (matching gh_plot_reports baseline). 2023 is
available via `--include-2023` on either scraper for future backfill.

corpus/agrigold_plot_reports/agpr-soybeans-2024-142209.md

@@ -0,0 +1,35 @@
+# Soybean yield trial — Palestine, IL, 2024
+
+- **Source:** AgriGold plot report (cross-vendor head-to-head)
+- **Vendor:** AgReliant Genetics / AgriGold
+- **Crop:** Soybean
+- **State:** IL
+- **County:** Crawford
+- **City:** Palestine
+- **Year:** 2024
+- **Plot ID:** 142209
+- **Cooperator:** Joel York
+- **Plot average:** 76.0 BU/Ac
+- **Planted:** 2024-04-21
+- **Harvested:** 2024-09-09
+- **Population:** 140,000 seeds/acre
+- **Row width:** 15.0"
+- **# Rows:** 22
+- **Soil type:** Loamy Sand
+- **Tillage:** No Till
+- **Previous crop:** Corn
+- **Irrigation:** Yes
+- **URL:** https://www.agrigold.com/soybeans/performance/soybean-yield-results/142209
+
+---
+## Results (by rank)
+
+| Rank | Brand | Product | Trait | Ck | H20 | Test Wt. | Yield | Adj Yield |
+|---|---|---|---|---|---|---|---|---|
+| 4 | AgriGold | G2893E3 | Enlist E3 | - | 11.6 | 56.0 | 75.0 | 75.0 |
+| 1 | AgriGold | G3051E3 | Enlist E3 | - | 11.4 | 56.0 | 78.3 | 78.3 |
+| 5 | AgriGold | G3279E3 | Enlist E3 | - | 13.2 | 55.5 | 73.1 | 73.1 |
+| 2 | NK Brand | NK33-Y7E3 | Enlist E3 | - | 13.8 | 58.0 | 76.9 | 76.9 |
+| 3 | AgriGold | G3577E3 | Enlist E3 | - | 13.6 | 59.0 | 76.7 | 76.7 |
+
+Top 5 by Yield: G3051E3 (AgriGold) 78.3, NK33-Y7E3 (NK Brand) 76.9, G3577E3 (AgriGold) 76.7, G2893E3 (AgriGold) 75.0, G3279E3 (AgriGold) 73.1.