chore(CI/CD): Prevent agents from running npm

[lstein]

Summary

Copilot, and presumably other coding agents, have a tendency to run npm when updating the front end code. This creates an unwanted package-lock.json file.

This PR does three things:

1. It adds an AGENTS.md file that gives coding agents explicit instructions to use pnpm rather than npm.
2. It adds a packageManager instruction to invokeai/frontend/web/scripts/package.json (which seemed to be missing)
3. It adds a check to the frontendchecks.yml workflow that will generate an error if package-lock.json is found in the PR.

Related Issues / Discussions

QA Instructions

1. Using a coding agent, ask it to perform a pnpm operation such as lint or build.
2. Confirm that it uses the proper tool.
3. Add a package-lock.json file to invokeai/frontend/web and try to push it into this PR. The frontend-checks check should fail.

Merge Plan

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)