Add content_publish manual and improve tool description ...

- Create manuals/content_publish.md with full usage guide,
  internal mechanics, supported types, and best practices
- Update ContentPublishTool description with usage guidelines
  and examples in the tool schema
- All tests pass (150 backend + 47 webclient)

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