Adds marketplace scaffolding (.claude-plugin/marketplace.json,
.github/plugin/marketplace.json) and a hello-world proof-of-concept
plugin to validate the format before migrating real skills.
Also fixes inventory.sh to include .agents/ in the hidden-dir exception
list so audits correctly surface the 16 existing skills.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
