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/lg_plot_reports/lgpr-silage-2024-143003.md

@@ -0,0 +1,28 @@
+# Silage yield trial — Ridgeway, Iowa, 2024
+
+- **Source:** LG Seeds plot report (top-5 cross-vendor)
+- **Vendor:** AgReliant Genetics / LG Seeds
+- **Crop:** Silage
+- **State:** Iowa (IA)
+- **City:** Ridgeway
+- **Year:** 2024
+- **Plot ID:** 143003
+- **Cooperator:** Dale Humpal
+- **Planted:** 2024-05-14
+- **Harvested:** 2024-09-11
+- **Plot average:** 30262.0 T/Ac
+- **Coordinates:** 43.25000, -91.92000
+- **URL:** https://lgseeds.com/performance/silage#plot-143003
+
+---
+## Top 5 results (by yield rank)
+
+| Rank | Brand | Product | Traits | Ton/Acre |
+|---|---|---|---|---|
+| 1 | Pioneer | P0924Q | - | 36691.0 |
+| 2 | LG Seeds | LG66C06 | VT2RIB | 33363.2 |
+| 3 | Unknown | LG63C54PCE | - | 33288.2 |
+| 4 | LG Seeds | LG58C48 | VT2RIB | 31968.1 |
+| 5 | Unknown | B11B31Q | - | 31671.0 |
+
+Top 5 by Ton/Acre: P0924Q (Pioneer) 36691.0, LG66C06 (LG Seeds) 33363.2, LG63C54PCE (Unknown) 33288.2, LG58C48 (LG Seeds) 31968.1, B11B31Q (Unknown) 31671.0.