Add Makefile and update startup.sh for improved environment setup

- Introduce a Makefile to define standard targets for installation, data preparation, plotting, testing, and cleaning.
- Update `startup.sh` to enhance clarity and functionality, including better command handling and virtual environment management.
- Document changes in `repo-context.txt` to reflect the addition of the Makefile.

Makefile

@@ -0,0 +1,29 @@
+# Makefile for python_charts
+# Defines standard targets for Codex/CI to use startup.sh
+
+.PHONY: install data plot test clean
+
+# Install dependencies into venv and prepare data
+install:
+	./startup.sh pip install
+
+# Download FRED data (if needed)
+data:
+	./startup.sh scripts/refresh_data.py
+
+# Generate the default chart (18-month lag)
+plot:
+	./startup.sh
+
+# Generate a custom chart	
+# Usage: make plot ARGS="--offset 12 --end 2025-05-31 --extend-years 5"
+plot-custom:
+	./startup.sh $(ARGS)
+
+# Run test suite
+test:
+	./startup.sh pytest -q
+
+# Remove virtual environment and outputs
+clean:
+	rm -rf venv data outputs