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>

docs_mcp/lessons.md

@@ -1,8 +1,8 @@
-# PPLS API Lessons
+# Crop-Chem API Lessons
 
 Curated agronomy + label-handling knowledge that an LLM should know
 *before* giving recommendations from the labels corpus. Surfaced by
-the `ppls_api_lessons` MCP tool.
+the `crop_chem_api_lessons` MCP tool.
 
 Each top-level `## Topic: <slug>` block is independently retrievable.
 The tool docstring tells the LLM to call this proactively before
@@ -12,7 +12,7 @@ answering any pesticide recommendation question.
 
 ## Topic: how-to-use-this-corpus
 
-The PPLS docs corpus is the source of truth for *what's on the label*.
+The crop-chem-docs label corpus is the source of truth for *what's on the label*.
 You should:
 
 1. **Run `search_docs` first** with the user's natural-language