fix(registry): rename URL template variable to dodge collision (#5)

v0.1.3 publish failed with:
  remote URL https://{host}/mcp is already used by
  server io.github.kgNatx/mockups-gallery

The MCP Registry treats remotes[].url's LITERAL template string as a
uniqueness key, not the post-substitution URL. {host} was claimed
first, so it's burned for everyone.

Renaming the variable to {ibkr_mcp_host} makes the literal template
string unique while keeping the post-substitution URL shape identical
('https://your-host/mcp'). No effect on actual deployments -- the path
'/mcp' still matches the server's hardcoded MCP_PATH.