Update README with Claude Desktop extension docs and clearer usage

Adds full Claude Desktop setup section explaining .mcpb format, install
from GitHub Releases, manual update process, and Connectors Directory
submission status. Clarifies Claude Code /agents command for verifying
install. Reorganizes into distinct Claude Code and Claude Desktop sections.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: taylorarndt