feat: add devcontainer for local setup

.devcontainer/scripts/post-start.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Intentionally no `http://localhost:PORT` URLs below — VS Code's terminal
+# URL detector adds any printed URL to its auto-forwarded-ports list and
+# then polls it, which produces ECONNREFUSED log spam every ~20s for ports
+# that aren't bound yet. The Ports panel auto-detects bound ports anyway.
+cat <<'EOF'
+Dograh devcontainer ready.
+
+Start the backend:
+  bash scripts/start_services_dev.sh
+
+Start the UI in another terminal:
+  cd ui && npm run dev -- --hostname 0.0.0.0
+
+URLs and other workflow notes: docs/contribution/setup.mdx
+EOF