feat: Introduce local MCP server for CLI interaction

[tizmagik]

WHY are these changes introduced?

This change introduces a local MCP server to facilitate interaction with the Shopify CLI.

This is a draft for now as we evaluate the value of a local MCP server vs just a Skill (or both) to facilitate AI agents developing Shopify Themes (and Apps, for that matter).

WHAT is this pull request doing?

The pull request implements a local MCP server that registers tools for authentication and command execution, facilitating the CLI's usability for LLM agents.

The MCP tools are generic enough so that they're not tightly coupled to each individual CLI command while having sensible guardrails and patterns for auth, command discovery and usage by an LLM.

How to test your changes?

1. Clone the repository and navigate to the mcp-server package.
2. Build the project using the provided scripts.
3. Run the local MCP server and test the authentication and command execution tools.

Post-release steps

No post-release steps required.

Measuring impact

• n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes

[github-actions]

We detected some changes at packages/*/src and there are no updates in the .changeset.
If the changes are user-facing, run pnpm changeset add to track your changes and include them in the next release CHANGELOG.

Caution

DO NOT create changesets for features which you do not wish to be included in the public changelog of the next CLI release.

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	79.31%	14472/18248
🟡	Branches	73.53%	7174/9757
🟡	Functions	79.52%	3689/4639
🟡	Lines	79.66%	13683/17177

Test suite run success

3718 tests passing in 1438 suites.

Report generated by 🧪jest coverage report action from f168d5b