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-2025-146905.md

@@ -0,0 +1,51 @@
+# Soybean yield trial — Litchfield, NE, 2025
+
+- **Source:** AgriGold plot report (cross-vendor head-to-head)
+- **Vendor:** AgReliant Genetics / AgriGold
+- **Crop:** Soybean
+- **State:** NE
+- **County:** Sherman
+- **City:** Litchfield
+- **Year:** 2025
+- **Plot ID:** 146905
+- **Cooperator:** Zoerb Farms
+- **Plot average:** 91.94 BU/Ac
+- **Planted:** 2025-04-29
+- **Harvested:** 2025-10-10
+- **Population:** 140,000 seeds/acre
+- **Row width:** 20.0"
+- **# Rows:** 24
+- **Soil type:** Silt Loam
+- **Tillage:** No Till
+- **Previous crop:** Corn
+- **Irrigation:** Yes
+- **URL:** https://www.agrigold.com/soybeans/performance/soybean-yield-results/146905
+
+---
+## Results (by rank)
+
+| Rank | Brand | Product | Trait | Ck | H20 | Test Wt. | Yield | Adj Yield |
+|---|---|---|---|---|---|---|---|---|
+| - | Pioneer | P28Z30E | Enlist E3 | - | 14.4 | - | 93.0 | 93.0 |
+| 15 | Pioneer | P20Z14E | Enlist E3 | - | 14.3 | - | 89.5 | 89.62 |
+| 17 | AgriGold | G2265E3 | - | - | 14.2 | - | 88.7 | 88.81 |
+| 1 | Pioneer | P22Z02E | Enlist E3 | - | 14.1 | - | 97.5 | 97.65 |
+| 13 | Pioneer | P23Z58E | Enlist E3 | - | 13.8 | - | 90.9 | 91.0 |
+| 4 | Pioneer | P23Z58E | Enlist E3 | - | 13.7 | - | 94.3 | 94.43 |
+| 16 | Connect | CT2424E | Enlist E3 | - | 13.3 | - | 89.0 | 89.06 |
+| 7 | AgriGold | G2425E3 | - | - | 13.5 | - | 92.4 | 92.48 |
+| 3 | Stine | 24EH32 | Enlist E3 | - | 12.9 | - | 94.6 | 94.63 |
+| 14 | Alloy | A25E36 | Enlist E3 | - | 12.9 | - | 90.9 | 90.95 |
+| 5 | Stine | 26EH02 | Enlist E3 | - | 13.4 | - | 94.1 | 94.06 |
+| 18 | Connect | CT2725E | Enlist E3 | - | 12.9 | - | 88.5 | 88.46 |
+| 19 | Stine | 27EG22 | Enlist E3 | - | 13.5 | - | 87.0 | 86.93 |
+| 8 | Pioneer | P27Z41E | Enlist E3 | - | 13.0 | - | 92.4 | 92.31 |
+| 11 | Pioneer | P28Z30E | Enlist E3 | - | 13.1 | - | 91.9 | 91.87 |
+| 9 | Pioneer | P28Z30E | Enlist E3 | - | 13.0 | - | 92.4 | 92.28 |
+| 10 | Pioneer | P28Z30E | Enlist E3 | - | 13.4 | - | 92.2 | 92.14 |
+| 12 | Alloy | A29E36 | Enlist E3 | - | 12.6 | - | 91.6 | 91.43 |
+| 6 | Stine | 30EH32 | Enlist E3 | - | 14.6 | - | 93.7 | 93.52 |
+| 2 | Pioneer | P31Z03E | Enlist E3 | - | 14.1 | - | 95.4 | 95.27 |
+| - | Pioneer | P28Z30E | Enlist E3 | - | 13.8 | - | 93.3 | 93.3 |
+
+Top 5 by Yield: P22Z02E (Pioneer) 97.5, P31Z03E (Pioneer) 95.4, 24EH32 (Stine) 94.6, P23Z58E (Pioneer) 94.3, 26EH02 (Stine) 94.1.