fix(release): auto-update Homebrew cask on release (#367)

Add two steps to the update-homebrew job:
- Download both darwin installer DMGs and compute their SHA256s
- Patch Casks/mcpproxy.rb with the new version and hashes

Previously only Formula/mcpproxy.rb was updated automatically;
the cask was stuck at v0.20.2 while the formula was at v0.23.1.

Fixes smart-mcp-proxy/mcpproxy-go#366

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>