feat: add publish-openclaw-skills.js to push skills to ClawHub (#273)

ClawHub does not sync from GitHub. Each OpenClaw skill is pushed with the
clawhub CLI at its own version, so the published copies can drift from the
repo the same way the plugin manifests did (#260). This adds a one-pass
publisher that pushes every generated .openclaw/skills/ skill at the
package.json version, with --dry-run to preview, and documents it in the
README next to the build step.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>