rename: ppls-docs → crop-chem-docs

Repo/project rename to better reflect scope. PPLS is EPA's term for
their Pesticide Product Label System — accurate when the corpus was
EPA-only, narrow now that it also pulls from Bayer's own catalog
(and may expand to Syngenta/Corteva/BASF/FMC labels in the future).
crop-chem-docs scopes flexibly without acronyms to explain.

Renames:
- directory:           ppls-docs            → crop-chem-docs
- PRODUCT_NAME:        ppls                 → crop_chem
- Chroma collection:   ppls_docs            → crop_chem_docs  (in-place via .modify(), no re-embed)
- BM25 db:             bm25/ppls_docs.db    → bm25/crop_chem_docs.db
- MCP tool name:       ppls_api_lessons     → crop_chem_api_lessons
- FastMCP server name: ppls-docs            → crop-chem-docs
- Env vars:            PPLS_CORPUS_ROOT     → CORPUS_ROOT
                       PPLS_CHROMA_DIR      → CHROMA_DIR_OVERRIDE
- User-Agent:          ppls-docs-scraper    → crop-chem-docs-scraper

Preserved (intentional, correct):
- epa_ppls (source id) — refers specifically to EPA's PPLS database
- "EPA PPLS" mentions in regulatory text (lessons.md, server docstrings)
- PPLS_API_BASE / PPLS_PDF_BASE / PPLS_INDEX_URL_TEMPLATE in
  scrape/sources/epa_ppls.py — these point at EPA's actual endpoints

Memory entries get updated in a follow-up commit so the rename is
isolated.

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

scrape/sources/bayer.py

@@ -47,7 +47,7 @@ import requests
 from pypdf import PdfReader
 
 SCRAPER_VERSION = "0.1.0"
-USER_AGENT = "ppls-docs-scraper/0.1 (+https://drawbar.example/contact)"
+USER_AGENT = "crop-chem-docs-scraper/0.1 (+https://drawbar.example/contact)"
 BASE = "https://www.cropscience.bayer.us"
 
 # Catalog product-type values used in the Next.js data API.
@@ -63,11 +63,11 @@ PRODUCT_CLASS = {
 }
 
 # Repo root: scrape/sources/bayer.py -> repo root is 3 parents up.
-# Corpus root is overridable via PPLS_CORPUS_ROOT for routing the
+# Corpus root is overridable via CORPUS_ROOT for routing the
 # corpus to external storage (USB drive, NAS mount, etc.) without
 # editing the repo.
 REPO_ROOT = Path(__file__).resolve().parents[2]
-CORPUS_ROOT = Path(os.environ.get("PPLS_CORPUS_ROOT") or REPO_ROOT / "corpus")
+CORPUS_ROOT = Path(os.environ.get("CORPUS_ROOT") or REPO_ROOT / "corpus")
 CORPUS_DIR = CORPUS_ROOT / "bayer"
 
 # Politeness: target ~1 req/sec to Bayer. Each HTTP method goes through