feat(agent-arka): add web-dashboard installer and instructions

Public curl|bash path for the daemon + browser UI at :4545.
Binaries ship as release assets on this repo (private agent-arka
source stays private).

install-agent-arka.sh

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+# Compatibility stub. Canonical installer:
+#   https://git.arka-ai.ru/arka/public/raw/branch/main/agent-arka/install.sh
+set -euo pipefail
+echo "==> installer lives at agent-arka/install.sh" >&2
+curl -fsSL \
+  'https://git.arka-ai.ru/arka/public/raw/branch/main/agent-arka/install.sh' \
+  | bash -s -- "$@"