WIP: prepare for hexstrike subtree

scripts/add_hexstrike_subtree.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+# Helper script to vendor HexStrike into this repo using git subtree.
+# Run from repository root.
+
+set -euo pipefail
+
+REPO_URL="https://github.com/0x4m4/hexstrike-ai.git"
+PREFIX="third_party/hexstrike"
+BRANCH="main"
+
+echo "This will add HexStrike as a git subtree under ${PREFIX}."
+echo "If you already have a subtree, use 'git subtree pull' instead.\n"
+
+git subtree add --prefix="${PREFIX}" "${REPO_URL}" "${BRANCH}" --squash
+
+echo "HexStrike subtree added under ${PREFIX}."