feat: add host-side interactive tests for macOS coverage

Add test/run-interactive.sh that runs tmux interactive tests on
the host in an isolated HOME. Covers macOS ssh-keygen which
cannot be tested in Linux containers. e2e.sh now runs host
interactive tests first, then container matrix. Skips gracefully
if tmux is not installed.

Closes: #23

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

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

test/run-interactive.sh

@@ -0,0 +1,52 @@
+#!/usr/bin/env bash
+# Run interactive tmux tests on the host in an isolated HOME.
+# Covers macOS ssh-keygen and platform-specific behavior that
+# cannot be tested inside Linux containers.
+#
+# Requires: tmux, git, ssh-keygen
+
+set -o errexit
+set -o nounset
+set -o pipefail
+IFS=$'\n\t'
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+readonly SCRIPT_DIR
+REPO_ROOT="$(cd "${SCRIPT_DIR}/.." && pwd)"
+readonly REPO_ROOT
+
+die() {
+    printf 'Error: %s\n' "$1" >&2
+    exit 1
+}
+
+# Check dependencies
+command -v tmux >/dev/null 2>&1 || die "tmux is required. Install with: brew install tmux"
+command -v git >/dev/null 2>&1 || die "git is required"
+command -v ssh-keygen >/dev/null 2>&1 || die "ssh-keygen is required"
+
+# Create isolated HOME
+TEST_HOME="$(mktemp -d)"
+trap 'rm -rf "$TEST_HOME"' EXIT
+
+# Set up the isolated environment
+export HOME="$TEST_HOME"
+export GIT_CONFIG_GLOBAL="${TEST_HOME}/.gitconfig"
+
+mkdir -p "${TEST_HOME}/.ssh"
+mkdir -p "${TEST_HOME}/.config/git"
+
+# Copy the script into the test home (interactive helpers expect it at ~/git-harden.sh)
+cp "${REPO_ROOT}/git-harden.sh" "${TEST_HOME}/git-harden.sh"
+
+# Copy interactive test scripts
+cp -r "${SCRIPT_DIR}/interactive" "${TEST_HOME}/test-interactive"
+
+# Set up minimal git config
+git config --global user.name "Test User"
+git config --global user.email "test@example.com"
+
+printf '── Running interactive tests on host (%s) ──\n' "$(uname -s)" >&2
+
+# Run the interactive tests
+exec bash "${TEST_HOME}/test-interactive/run-all.sh"