Add an .actrc, runner Dockerfile, push event payload, and
scripts/dev/act_ci.sh to run ci.yml jobs locally with nektos/act.
The script refuses --bind/--reuse in .actrc, since a bound checkout
runs as root and rewrites ownership and modes in the working tree.
Document the @latest-v2 version label in the README.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
