refactor: move OpenCode artifacts under opencode/

arka/public is the dump for all Arka downloads. Keep a root stub so the
old installer URL still works. Document in-OpenCode bash install.

opencode/scripts/cron-sync.sh

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+# Daily catalog refresh for arka/public/opencode.
+# Runs on core at 05:00 Europe/Moscow.
+set -euo pipefail
+
+# this file lives at opencode/scripts/cron-sync.sh → repo root is ../..
+REPO="$(cd "$(dirname "$0")/../.." && pwd)"
+cd "$REPO"
+
+export GIT_SSH_COMMAND="${GIT_SSH_COMMAND:-ssh -i /root/.ssh/arka_public_catalog -o IdentitiesOnly=yes -o StrictHostKeyChecking=accept-new}"
+
+git fetch origin main
+git checkout -q main
+git reset --hard origin/main
+
+python3 opencode/scripts/sync-catalog.py
+
+if git diff --quiet -- opencode/catalog.json; then
+  printf '%s no catalog change\n' "$(date -u +%Y-%m-%dT%H:%M:%SZ)"
+  exit 0
+fi
+
+git add opencode/catalog.json
+git -c user.name='arka-catalog-bot' -c user.email='noreply@arka-ai.ru' \
+  commit -m "chore(catalog): refresh $(date -u +%Y-%m-%d)"
+git push origin main
+printf '%s catalog pushed\n' "$(date -u +%Y-%m-%dT%H:%M:%SZ)"