Add root CLAUDE.md as navigation hub

- Points to both R (legacy) and Python (active) projects
- Links to detailed Python documentation in a4d-python/docs/
- Warns about shared reference_data/ used by both
- Ensures AI assistance can find guidance at repository root

Author: pmayd

CLAUDE.md

@@ -0,0 +1,59 @@
+# CLAUDE.md
+
+This repository contains **two projects**:
+
+## 1. R Pipeline (Production - Legacy)
+
+**Location**: Root directory
+**Status**: Production (being phased out)
+
+The original R implementation of the A4D medical tracker data processing pipeline.
+
+**Key Files**:
+- `R/` - R package code
+- `scripts/R/` - Pipeline scripts
+- `reference_data/` - Shared YAML configurations
+
+**Commands**: See README.md for R-specific commands
+
+---
+
+## 2. Python Pipeline (Active Development)
+
+**Location**: `a4d-python/`
+**Status**: Active migration
+**Branch**: `migration`
+
+New Python implementation with better performance and incremental processing.
+
+**Documentation**: [a4d-python/docs/CLAUDE.md](a4d-python/docs/CLAUDE.md)
+
+**Quick Start**:
+```bash
+cd a4d-python
+uv sync
+uv run pytest
+```
+
+**Migration Guide**: [a4d-python/docs/migration/MIGRATION_GUIDE.md](a4d-python/docs/migration/MIGRATION_GUIDE.md)
+
+---
+
+## Working on This Repository
+
+**If working on R code**: Stay in root, use R commands
+
+**If working on Python migration**:
+```bash
+cd a4d-python
+# See a4d-python/docs/CLAUDE.md for Python-specific guidance
+```
+
+## Shared Resources
+
+Both projects use the same reference data:
+- `reference_data/synonyms/` - Column name mappings
+- `reference_data/data_cleaning.yaml` - Validation rules
+- `reference_data/provinces/` - Allowed provinces
+
+**Do not modify these** without testing both R and Python pipelines.