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-2025-145465.md

@@ -0,0 +1,43 @@
+# Soybean yield trial — Princeton, IL, 2025
+
+- **Source:** AgriGold plot report (cross-vendor head-to-head)
+- **Vendor:** AgReliant Genetics / AgriGold
+- **Crop:** Soybean
+- **State:** IL
+- **County:** Bureau
+- **City:** Princeton
+- **Year:** 2025
+- **Plot ID:** 145465
+- **Cooperator:** Alan Madison
+- **Plot average:** 85.48 BU/Ac
+- **Planted:** 2025-04-24
+- **Harvested:** 2025-09-28
+- **Population:** 110,000 seeds/acre
+- **Row width:** 30.0"
+- **# Rows:** 8
+- **Soil type:** Silt Loam
+- **Tillage:** Strip Till
+- **Previous crop:** Corn
+- **Irrigation:** None
+- **URL:** https://www.agrigold.com/soybeans/performance/soybean-yield-results/145465
+
+---
+## Results (by rank)
+
+| Rank | Brand | Product | Trait | Ck | H20 | Test Wt. | Yield | Adj Yield |
+|---|---|---|---|---|---|---|---|---|
+| 11 | AgriGold | G2601E3 | Enlist E3 | - | 10.9 | 57.7 | 80.9 | 80.9 |
+| 9 | FS Seeds | HS26E20 | Enlist E3 | - | 10.4 | 60.6 | 84.9 | 84.9 |
+| 6 | FS Seeds | HS28E10 | Enlist E3 | - | 10.4 | 59.0 | 87.1 | 87.1 |
+| 4 | Pioneer | P28Z30E | Enlist E3 | - | 10.2 | 59.3 | 89.4 | 89.4 |
+| 7 | Pioneer | P28Z89E | Enlist E3 | - | 10.3 | 58.9 | 86.6 | 86.6 |
+| 2 | Pioneer | P29Z61E | Enlist E3 | - | 10.4 | 58.3 | 90.3 | 90.3 |
+| 8 | AgriGold | G3051E3 | Enlist E3 | - | 10.6 | 58.2 | 85.8 | 85.8 |
+| 3 | Pioneer | P31Z03E | Enlist E3 | - | 10.7 | 57.7 | 90.2 | 90.2 |
+| 10 | FS Seeds | HS31E20 | Enlist E3 | - | 10.3 | 58.0 | 82.5 | 82.5 |
+| 5 | Pioneer | P31Z32E | Enlist E3 | - | 10.2 | 59.9 | 87.3 | 87.3 |
+| 1 | Pioneer | P33Z17E | Enlist E3 | - | 10.2 | 59.4 | 92.2 | 92.2 |
+| 13 | FS Seeds | HS34E40 | Enlist E3 | - | 10.4 | 61.4 | 74.8 | 74.8 |
+| 12 | AgriGold | G3425E3 | Enlist E3 | - | 10.1 | 59.2 | 79.3 | 79.3 |
+
+Top 5 by Yield: P33Z17E (Pioneer) 92.2, P29Z61E (Pioneer) 90.3, P31Z03E (Pioneer) 90.2, P28Z30E (Pioneer) 89.4, P31Z32E (Pioneer) 87.3.