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/interactive/run-all.sh

@@ -0,0 +1,30 @@
+#!/usr/bin/env bash
+# Run all interactive tmux-driven tests
+# Intended to be run inside a container (with tmux installed)
+
+set -o nounset
+set -o pipefail
+IFS=$'\n\t'
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+
+passed=0
+failed=0
+total=0
+
+for test_script in "${SCRIPT_DIR}"/test-*.sh; do
+    [ -f "$test_script" ] || continue
+    total=$((total + 1))
+    printf '\n── %s ──\n' "$(basename "$test_script")" >&2
+    if bash "$test_script"; then
+        passed=$((passed + 1))
+    else
+        failed=$((failed + 1))
+    fi
+done
+
+printf '\n── Interactive tests: %d passed, %d failed, %d total ──\n' "$passed" "$failed" "$total" >&2
+
+if [ "$failed" -gt 0 ]; then
+    exit 1
+fi