feat: add e2e container test harness

Implements spec docs/specs/2026-03-30-e2e-container-tests.md:
- 5 Containerfiles (ubuntu, debian, fedora, alpine, arch)
- test/e2e.sh runner with --runtime, --rebuild, single-distro mode
- tmux-based interactive tests (full accept, safety gate decline,
  signing generate, signing skip)
- All scripts pass shellcheck

Closes: #18, #19, #20

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

test/containers/Containerfile.arch

@@ -0,0 +1,22 @@
+FROM archlinux:base
+
+RUN pacman -Syu --noconfirm \
+        bash \
+        git \
+        openssh \
+        tmux \
+    && pacman -Scc --noconfirm
+
+RUN useradd -m -s /bin/bash testuser
+
+COPY git-harden.sh /home/testuser/git-harden.sh
+COPY test/ /home/testuser/test/
+RUN chown -R testuser:testuser /home/testuser
+
+USER testuser
+WORKDIR /home/testuser
+
+RUN git config --global user.name "Test User" \
+    && git config --global user.email "test@example.com"
+
+CMD ["bash", "test/run.sh"]