fix: rename core to spec

[baxen]

Renaming core to spec to clarify organization (#6) and to avoid naming conflicts on crates.io

[stevohuncho]

I run the mcp-core crate and was wondering if we could collaborate and solve this issue so this could be the official mcp-core

[RWDai]

I run the mcp-core crate and was wondering if we could collaborate and solve this issue so this could be the official mcp-core我运行了 mcp-core crate，想知道我们能否合作解决这个问题，这样就可以成为官方的 mcp-core

you delete you mcp-core package？

[cbrit]

Somewhat related, I mentioned in #17 using generated bindings from the official spec's JSON schema rather than the current hand-rolled types. I've published a simple version at https://crates.io/crates/mcp-types, if there is interest, happy to remove so the name can be used. mcp-spec is nice too.

[stevohuncho]

@baxen I'm the author of that crate and I don't mind changing it as long as all of these crates will be live on crates.io and this project is production ready as I currently run remote SSE mcp servers with my crate.