fix(marketplace): drop duplicate skills spec to resolve manifest conflict

Both marketplace.json (plugin entry) and skills/.claude-plugin/plugin.json
declared `skills`, which newer Claude Code flags as conflicting manifests
under strict:false, blocking `/plugin install ppt-master@ppt-master`.

Remove the marketplace-side `skills`/`strict` so plugin.json is the sole
component source: marketplace entry now only owns source + metadata.

Fixes #133

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>