feat: add community marketplaces and improve schema flexibility

Add two new community marketplace sources to expand plugin ecosystem:
- claude-code-workflows (wshobson/agents): 62 plugins, 84 agents, 42 tools
- claude-code-templates (davila7/claude-code-templates): ready-to-use templates

Improve marketplace schema to support multiple JSON formats:
- Accept both root-level and metadata-nested version/description
- Make owner.email and owner.url optional for flexibility
- Normalize data with sensible defaults ('0.0.0', 'No description available')
- Remove redundant metadata field after transformation
- Add documentation explaining dual format support

Also remove duplicate marketplace-sources.json from public directory,
consolidating to single source in server directory for API consumption.

Author: amondnet