feat: OpenCode Arka-AI installer and daily chat catalog

Public one-liner for opencode. Catalog is the filtered chat list from
api.arka-ai.ru/v1/models, refreshed daily at 05:00 Europe/Moscow.

scripts/cron-sync.sh

@@ -0,0 +1,25 @@
+#!/usr/bin/env bash
+# Daily catalog refresh for arka/public. Runs on core at 05:00 Europe/Moscow.
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+cd "$ROOT"
+
+export GIT_SSH_COMMAND="${GIT_SSH_COMMAND:-ssh -i /root/.ssh/arka_public_catalog -o IdentitiesOnly=yes -o StrictHostKeyChecking=accept-new}"
+
+git fetch origin main
+git checkout -q main
+git reset --hard origin/main
+
+python3 scripts/sync-catalog.py
+
+if git diff --quiet -- catalog.json; then
+  printf '%s no catalog change\n' "$(date -u +%Y-%m-%dT%H:%M:%SZ)"
+  exit 0
+fi
+
+git add 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)"