Add a new `brew mcp-server` command for a Model Context Protocol (MCP)
server for Homebrew. This integrates with AI/LLM tools like Claude,
Claude Code and Cursor.
It currently supports the calls needed/used by the MCP Inspector and
Cursor (where I've tested it).
It provides as `tools` the subcommands output by `brew help` but should
be fairly straightforward to add more in future.
It is implemented in a slightly strange way (a standalone Ruby command
called from a shell command) as MCP servers need a faster startup time
than a normal Homebrew Ruby command allows and fail if they don't get
it.
There are a few Ruby libraries available but, given how relatively
simplistic the implementation is, it didn't feel worthwhile to use and
vendor them.
