test: add isolated install verification scripts

- Dockerfile.test: Ubuntu 24.04 container for clean-room setup testing
- test_isolated.sh: mock-based test verifying auto-install URLs are called

Dockerfile.test

@@ -0,0 +1,32 @@
+FROM ubuntu:24.04
+
+# Avoid interactive prompts during package installation
+ENV DEBIAN_FRONTEND=noninteractive
+
+# Install basic dependencies
+RUN apt-get update && apt-get install -y \
+    curl \
+    bash \
+    python3 \
+    python3-pip \
+    git \
+    sudo \
+    && rm -rf /var/lib/apt/lists/*
+
+# Install Node.js 22.x
+RUN curl -fsSL https://deb.nodesource.com/setup_22.x | bash - \
+    && apt-get install -y nodejs \
+    && rm -rf /var/lib/apt/lists/*
+
+# Create a non-root user (setup script refuses to run as root)
+RUN useradd -m -s /bin/bash tester
+
+# Switch to tester user
+USER tester
+WORKDIR /home/tester
+
+# Copy project files
+COPY --chown=tester:tester . /home/tester/project/
+
+# Run setup script
+CMD ["bash", "/home/tester/project/claude_setup.sh"]