chore: validate CODEOWNERS in CI and act

Split workflow into validate (PR + act) and update (dev bot PR). Add
structure checks per GitHub docs, API error validation, and act config.

scripts/test-codeowners-act.sh

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+cd "$(dirname "$0")/.."
+
+echo "==> Local CODEOWNERS checks"
+pnpm exec tsx ./scripts/generate-codeowners.mts --check
+export GITHUB_REPOSITORY="${GITHUB_REPOSITORY:-homarr-labs/homarr}"
+if [ -z "${GITHUB_TOKEN:-}" ] && [ -z "${GH_TOKEN:-}" ]; then
+  GITHUB_TOKEN="$(gh auth token 2>/dev/null || true)"
+  export GITHUB_TOKEN
+fi
+pnpm exec tsx ./scripts/validate-codeowners.mts
+
+echo "==> act: validate job (requires Docker)"
+act workflow_dispatch -W .github/workflows/update-codeowners.yml -j validate \
+  -s GITHUB_TOKEN="${GITHUB_TOKEN:-}"