fix: make demo setup/teardown idempotent and guard on PATH

Two failure modes found while validating the harness against a scratch
config root:

Teardown removed $dest unconditionally before restoring the backup, so
a second run deleted the user's own file — the first run had already
restored it, and a restored original is indistinguishable from an
installed demo file except by content. Compare against the source
before removing.

Setup had the mirror-image bug: run twice without an intervening
teardown it backed up its *own* copy, and teardown then "restored"
that, leaving demo.md in the user's config permanently. Treat an
already-identical destination as a no-op.

Also fail fast when bot-bottle is not on PATH. The tape types the real
console script, so without it a recording silently captures
`command not found` instead of a bottle launching.

Refs #540

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

scripts/demo-setup.sh

@@ -24,6 +24,16 @@ if ! docker info >/dev/null 2>&1; then
   exit 1
 fi
 
+# The tape types `bot-bottle start demo` verbatim, so the console
+# script has to resolve in the recording shell. Without this guard a
+# recording silently captures `command not found` instead of a bottle.
+if ! command -v bot-bottle >/dev/null 2>&1; then
+  echo "demo-setup: bot-bottle is not on PATH. The demo runs the real" >&2
+  echo "  console script, not ./cli.py — install it first (bash install.sh," >&2
+  echo "  or 'pip install -e .' into an active venv), then re-run." >&2
+  exit 1
+fi
+
 config_root="${BOT_BOTTLE_ROOT:-$HOME/.bot-bottle}"
 mkdir -p "$config_root/bottles" "$config_root/agents"
 
@@ -34,6 +44,13 @@ mkdir -p "$config_root/bottles" "$config_root/agents"
 install_demo_file() {
   src=$1
   dest=$2
+  # Already installed (setup run twice without an intervening
+  # teardown). Backing up our own copy here would make teardown
+  # "restore" it and leave the demo file in the user's config forever,
+  # so treat this as a no-op.
+  if [ -e "$dest" ] && cmp -s "$src" "$dest"; then
+    return 0
+  fi
   if [ -e "$dest.demo-backup" ]; then
     echo "demo-setup: $dest.demo-backup already exists — a previous run" >&2
     echo "  did not tear down cleanly. Inspect it, then remove or restore" >&2