Adds .omp-plugin/marketplace.json as the preferred catalog location for omp
marketplaces. fetchMarketplace now searches .omp-plugin/marketplace.json
first and falls back to .claude-plugin/marketplace.json so existing
Claude Code-compatible marketplaces keep loading.
Docs and authoring skill updated to reflect the new preferred path.
Co-Authored-By: omp <noreply@oh-my-pi.dev>