feat: add keyword filters to md.search (fixes #54)

[H-2006]

Summary

This PR adds keyword filter support to md.search while maintaining backward compatibility with the existing string query format. Fixes #54.

Changes

• Updated md.search in src/modeldock/__init__.py to accept category, capability, and min_ram arguments.
• Updated RegistryService.search in src/modeldock/core/registry.py to apply these filters to the result set.
• Added unit tests in tests/unit/test_search_filters.py.

Testing

• Verified backward compatibility with string-only queries.
• Verified filtering logic for category, capability, and min_ram.
• All tests passed locally via pytest tests/unit/test_search_filters.py.

Checklist

• Branch named per Git Workflow
• Not developed on main
• Code follows AGENT.md coding standards
• Quality gates pass locally: pytest