test(m1): assert plugin manifest and pin a build stamp

scripts/build_stamp.sh

@@ -0,0 +1,52 @@
+#!/bin/bash
+# Write the short hash of the commit this build came from to BUILD_STAMP.
+#
+# bash 3.2-clean and ASCII-only on purpose: the system bash on this Mac is 3.2.
+#
+# Why a stamp and not the manifest version: Cowork caches an uploaded plugin,
+# and plugin.json's version does not change between milestones, so a stale
+# build would echo the right number and every answer after it would be about
+# some other build (risk H11). The short hash moves on every commit, which is
+# exactly the property the check needs.
+#
+# The stamp is a build artefact and is gitignored. It is regenerated by
+# scripts/package_plugin.sh before every archive; a committed stamp would be
+# false the instant the next commit moved HEAD.
+#
+# Usage:
+#   bash scripts/build_stamp.sh                 # write <plugin root>/BUILD_STAMP
+#   bash scripts/build_stamp.sh --ut <sti>      # write somewhere else
+
+set -eu
+
+SCRIPT_DIR=$(cd "$(dirname "$0")" && pwd)
+REPO_ROOT=$(cd "$SCRIPT_DIR/.." && pwd)
+OUT="${REPO_ROOT}/BUILD_STAMP"
+
+while [ $# -gt 0 ]; do
+  case "$1" in
+    --ut)
+      if [ $# -lt 2 ]; then
+        echo "build_stamp: --ut needs a path" >&2
+        exit 2
+      fi
+      OUT="$2"
+      shift 2 ;;
+    -h|--help)
+      sed -n '2,20p' "$0"
+      exit 0 ;;
+    *)
+      echo "build_stamp: unknown argument: $1" >&2
+      exit 2 ;;
+  esac
+done
+
+if ! git -C "$REPO_ROOT" rev-parse --git-dir >/dev/null 2>&1; then
+  echo "build_stamp: $REPO_ROOT is not a git working tree." >&2
+  echo "build_stamp: the stamp is the commit this build came from; there is nothing to stamp." >&2
+  exit 1
+fi
+
+STAMP=$(git -C "$REPO_ROOT" rev-parse --short HEAD)
+printf '%s\n' "$STAMP" > "$OUT"
+echo "build_stamp: $STAMP -> $OUT"