feat(proposal): adopt vendor-neutral harness and support for open source models

## Summary

Currently the `gemara-mcp` server holds the resources, tools, and prompts for artifact creation. The main benefits of the mcp server are the build and validate capabilities that reference the lexicon. The `gemara-ai` repo has an open [PR](https://github.com/gemaraproj/gemara-ai/pull/6) which packages the `gemara-mcp` server as a claude code plugin alongside a couple skills. Namely, triaging by identifying what artifacts already exist in the repo and what the next logical artifact would be (e.g., `control-catalog.yaml` would logically be followed by a `policy.yaml`).

Users looking to test the functionalities in the `gemara-mcp` server and using the `gemara-ai` repo will _need_ to leverage an AI Agent. Ideally, we should support open source models and vendor-neutral harnesses.

## Goal

Adopt vendor-neutral harness and support for open source models. Create valuable content in the form of tutorials and how-to guides to ensure accessible communication and proper display of repeatability.

## Context from Discussions

* Discussed during the [07/23 Gemara bi-weekly meeting](https://docs.google.com/document/d/1Yi0dt-IC7rcIaja8d1gKj0m2ifbcIIbizA84JnLdKp4/edit?tab=t.0)
* Testing `gemara-ai` PR discussion = running into rate-limitations when using workarounds for claude code plugin

Start by reviewing the gemara-ai PR #6 and the gemara-mcp server context, then read the linked meeting notes and testing discussion. The issue needs a defined vendor-neutral harness, open-source model support, and a scoped set of tutorials or how-to guides; done should include repeatable guidance for testing the server.

Written by the indexing model from the issue text.