fix: restructure plugin system for Claude Code compatibility

Major fixes for Claude Code plugin installation:

1. Moved marketplace.json to repo root (.claude-plugin/)
   - Fixes known bug #11243, #11278 with local marketplace path resolution
   - Claude Code uses marketplace.json file path as base, not directory

2. Simplified plugin.json manifests
   - Removed commands/agents/skills directory declarations
   - Claude Code auto-discovers these from standard directories
   - Specifying directories caused "must end with .md" validation errors

3. Cleaned command frontmatter
   - Removed non-standard fields (name, agent, arguments)
   - Only description field is valid in command frontmatter

4. Removed deprecated claude-plugin-developer skill
   - Skill was never properly integrated
   - Removed from repo and all documentation references

All three plugins now validate successfully:
- projman: Sprint planning with Gitea/Wiki.js
- cmdb-assistant: NetBox CMDB integration
- project-hygiene: Post-task cleanup hooks

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -204,12 +204,6 @@ support-claude-mktplace/
 
 ## Development
 
-### Skills
-
-The repository includes a Claude Code skill for plugin development:
-
-- **[claude-plugin-developer](./.claude/skills/claude-plugin-developer/README.md)** - Expert guidance for creating secure, standards-compliant plugins
-
 ### Adding New Plugins
 
 1. Create plugin directory in `plugins/` with `.claude-plugin/plugin.json`