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>

.agents/skills/marketplace-architect/scripts/inventory.sh

@@ -33,7 +33,7 @@ while IFS= read -r -d '' path; do
   skip=false
   IFS='/' read -ra parts <<< "$dir"
   for part in "${parts[@]}"; do
-    if [[ "$part" == .* && "$part" != ".claude-plugin" && "$part" != ".github" ]]; then
+    if [[ "$part" == .* && "$part" != ".claude-plugin" && "$part" != ".github" && "$part" != ".agents" ]]; then
       skip=true; break
     fi
   done