Name the root command after the installed binary

The cobra root command is declared as "server", but the binary is built and
distributed as "github-mcp-server". Two user-visible consequences:

  * "github-mcp-server completion <shell>" emits a completion dispatcher
    keyed on "server" (_server, __start_server, ...). Installed as
    completions for github-mcp-server they never fire, because the function
    names and the final "complete ... server" registration refer to a
    command that is not on the user's PATH.

  * "--help" prints "Usage: server [command]", naming a command that does
    not exist.

Name the root command after the binary. cmd/mcpcurl already does this
correctly (Use: "mcpcurl"). Nothing else in the tree refers to the root
command by name, and no test asserts it.