fix(install): friendly bash guard instead of cryptic pipefail error under sh

agent-arka/install.sh

@@ -11,6 +11,14 @@
 #   AGENT_ARKA_INIT          1 to run `agent-arka init --quick` when no config.toml exists (default: 1)
 # LIBREFANG_* names for the same keys still work.
 
+# Fail with a clear hint when piped into POSIX `sh` (dash) — this script needs
+# bash (pipefail, [[ ]], indirect expansion). `curl … | bash`, not `| sh`.
+if [ -z "${BASH_VERSION:-}" ]; then
+  echo "Agent Arka installer needs bash. Run:" >&2
+  echo "  curl -fsSL https://git.arka-ai.ru/arka/public/raw/branch/main/agent-arka/install.sh | bash" >&2
+  exit 1
+fi
+
 set -euo pipefail
 
 PUBLIC_API="${AGENT_ARKA_PUBLIC_API:-https://git.arka-ai.ru/api/v1}"