[FEATURE] Create MCP package for stackql

Feature Description

Create a package inside the pkg directory, called mcp_server. This will have zero dependencies on stackql itself and it will contain:

1. An interface abstraction for the backend that will run queries arising from client requests to a backend, in the first instance this backend will be stackql, but we will do the implementation later in core code. This interface should be equally amenable to passing queries in memory or over TCP or other sockets.
2. Some config structure with json and yaml attribute definnitions to configure mcp_sefver.
3. An MCP server, probably using the golang mcp sdk. This should depend on the interface described in (1) and the config described in (2) and expose all conventional MCP transports, configurably.

The goal is for LLMS to consume stackql as a first class information source.

Example(s)

• I don't really want to go into examples at this stage. At the risk of being repetitive, the goal is for LLMs to consume stackql as a first class information source.

Possible Approaches or Libraries to Consider

• https://github.com/modelcontextprotocol/go-sdk

Additional context

Start by reviewing the existing pkg directory and the linked Model Context Protocol Go SDK. Define the boundaries for pkg/mcp_server: a transport-independent backend interface, JSON/YAML configuration, and an MCP server exposing the conventional configurable transports without depending on stackql. Done means the package builds with those abstractions and has no dependency on stackql itself.