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.

repo-context.txt

@@ -12,6 +12,7 @@ Generated on: 2025-05-30
         ├── lagged_oil_unrate_chart_styled.py
         ├── pyproject.toml
         ├── refresh_data.py
+    ├── Makefile
     ├── README.md
     ├── requirements.txt
     ├── startup.sh
@@ -176,15 +177,13 @@ python-dateutil>=2.8.1
 ```
 #!/usr/bin/env bash
 # -------------------------------------------------------------------
-# startup.sh  — prepare Python env, ensure dependencies, FRED data,
-#              and run a command inside the venv
+# startup.sh  — prepare Python env, ensure deps & data, then run.
 # Usage:
-#   ./startup.sh [command…]
-# Automatically:
-#  - Upgrades pip & setuptools (ensuring distutils on Python 3.12+)
-#  - Creates venv if missing
-#  - Installs requirements
-#  - Downloads data/fred.db if missing
+#   ./startup.sh [options]      # passes options to default plot script
+#   ./startup.sh [script or cmd] # runs specified script/command
+# Examples:
+#   ./startup.sh --offset 12 --end 2025-05-31 --extend-years 5
+#   ./startup.sh pytest -q
 # -------------------------------------------------------------------
 set -euo pipefail
 
@@ -192,49 +191,94 @@ VENV_DIR="venv"
 REQ_FILE="requirements.txt"
 WHEELHOUSE="wheelhouse"
 DATA_DB="data/fred.db"
+PYTHON="$VENV_DIR/bin/python"
+DEFAULT_SCRIPT="scripts/lagged_oil_unrate_chart_styled.py"
 
 # 1) Create or reuse venv
 if [[ ! -d "$VENV_DIR" ]]; then
   echo "🔧 Creating virtual-env…"
   python3 -m venv "$VENV_DIR"
 fi
-# Activate
+
+# 2) Activate venv
 # shellcheck disable=SC1090
 source "$VENV_DIR/bin/activate"
 
-# 2) Upgrade pip & setuptools (to provide distutils on Py3.12+)
+# 3) Upgrade pip & setuptools
 echo "🛠 Upgrading pip & setuptools…"
-pip install --upgrade pip setuptools
+$PYTHON -m pip install --upgrade pip setuptools
 
-# 3) Install project dependencies
+# 4) Install dependencies
 PIP_OPTS=(install --upgrade --no-input)
 if [[ -d "$WHEELHOUSE" ]]; then
   echo "📦 Installing from wheelhouse…"
-  pip "${PIP_OPTS[@]}" --no-index --find-links="$WHEELHOUSE" -r "$REQ_FILE" \
-    || { echo "⚠️ Wheelhouse failed—falling back to PyPI…"; \
-         pip "${PIP_OPTS[@]}" -r "$REQ_FILE"; }
+  $PYTHON -m pip "${PIP_OPTS[@]}" --no-index --find-links="$WHEELHOUSE" -r "$REQ_FILE" \
+    || { echo "⚠️ Wheelhouse failed—falling back…"; \
+         $PYTHON -m pip "${PIP_OPTS[@]}" -r "$REQ_FILE"; }
 else
   echo "🌐 Installing from PyPI…"
-  pip "${PIP_OPTS[@]}" -r "$REQ_FILE"
+  $PYTHON -m pip "${PIP_OPTS[@]}" -r "$REQ_FILE"
 fi
 
-echo "✅ Virtual-env ready ($(python -V))"
+echo "✅ Virtual-env ready ($($PYTHON -V))"
 
-# 4) Ensure data/fred.db exists
+# 5) Ensure data exists
 if [[ ! -f "$DATA_DB" ]]; then
-  echo "🔄 $DATA_DB not found—downloading FRED series…"
-  python scripts/refresh_data.py
+  echo "🔄 Data not found—downloading FRED series…"
+  $PYTHON scripts/refresh_data.py
 fi
 
-# 5) Decide what to run
-if [[ $# -gt 0 ]]; then
-  CMD=("$@")
+# 6) Build command
+if [[ $# -eq 0 ]]; then
+  CMD=("$PYTHON" "$DEFAULT_SCRIPT" --offset 18)
+elif [[ "$1" == --* ]]; then
+  CMD=("$PYTHON" "$DEFAULT_SCRIPT" "$@")
 else
-  CMD=(python scripts/lagged_oil_unrate_chart_styled.py --offset 18)
+  if [[ -f "$1" || "$1" == *.py ]]; then
+    CMD=("$PYTHON" "$@")
+  else
+    CMD=("$@")
+  fi
 fi
 
+# 7) Execute
 echo "🚀 Executing: ${CMD[*]}"
 exec "${CMD[@]}"
+
+```
+
+## python_charts/Makefile
+```
+# 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
+
 ```
 
 ## scripts/refresh_data.py