feature: Change instructions at runtime

[0xNF]

Currently, the Instructions field of the MCPServer must be set during construction:

options := []server.ServerOption{
	server.WithInstructions("..."),
}
s := server.NewMCPServer(serverName, version, options...)

I have instructions that can theoretically change over the lifetime of the server, and to the best of my knowledge, there's no way to modify instructions after the object has been instantiated.

I'd like some kind of server.SetInstructions(instructions string) method.