docs: add end-to-end demo with recorded GIF

scripts/demo.sh + scripts/demo_harness.py drive a real bottle through
four probes (pipelock allow, host-allowlist block, DLP body-scan
block, git-gate gitleaks rejection). docs/demo.tape is the VHS source
that renders docs/demo.gif, embedded at the top of the README as a
working proof of the security model the prose describes.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

scripts/demo.sh

@@ -0,0 +1,56 @@
+#!/usr/bin/env bash
+# Demo runner: builds the image graph if needed, then runs the four-scenario
+# harness against a real bottle. Designed to produce screen-recordable
+# output — paced banners, color, no Python tracebacks unless something
+# actually breaks.
+#
+# Usage:
+#   bash scripts/demo.sh                  # run live
+#   vhs docs/demo.tape                    # record to docs/demo.gif
+
+set -euo pipefail
+
+cd "$(dirname "$0")/.."
+
+verbose=0
+for arg in "$@"; do
+  case "$arg" in
+    -v|--verbose) verbose=1 ;;
+    -h|--help)
+      cat <<EOF
+Usage: bash scripts/demo.sh [--verbose]
+
+Runs four pipelock + git-gate probes against a real bottle and prints
+PASS/BLOCK verdicts. Without --verbose, Docker build chatter and
+backend log lines are suppressed so the output is recordable.
+EOF
+      exit 0 ;;
+  esac
+done
+
+if ! command -v docker >/dev/null 2>&1; then
+  echo "docker not found on PATH — install Docker Desktop or equivalent first" >&2
+  exit 1
+fi
+
+if ! docker info >/dev/null 2>&1; then
+  echo "docker daemon not reachable — start Docker and re-run" >&2
+  exit 1
+fi
+
+# Pre-warm the image graph quietly so the recorded run shows only the
+# four scenario blocks, not BuildKit progress. The backend rebuilds
+# (cache-hit) on launch regardless; doing it once up front keeps the
+# launch-time chatter short.
+if [ "$verbose" = 0 ]; then
+  docker build -q -t claude-bottle:latest . >/dev/null 2>&1 || true
+  docker build -q -f Dockerfile.git-gate -t claude-bottle-git-gate:latest . >/dev/null 2>&1 || true
+fi
+
+if [ "$verbose" = 1 ]; then
+  exec python3 -u scripts/demo_harness.py
+else
+  # Stderr carries backend info() lines and BuildKit chatter; drop it.
+  # The harness writes all scenario output (banners, results) to stdout.
+  exec python3 -u scripts/demo_harness.py 2>/dev/null
+fi