feat: bootstrap holocron marketplace with hello-world plugin

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>

plugins/hello-world/plugin.json

@@ -0,0 +1,4 @@
+{
+  "name": "hello-world",
+  "description": "A minimal example plugin to validate marketplace scaffolding."
+}