Fix only-pnpm to support pnpm 11, and dedupe script

[CraigMacomber]

Description

Fix only-pnpm to support pnpm 11, and dedupe script

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[Copilot]

Pull request overview

This PR updates pnpm enforcement logic and removes a duplicated compatibility-workspace copy of the pnpm-only install guard.

Changes:

• Updates the root only-pnpm.cjs check to use npm_execpath.
• Removes the compat-workspaces-local copy of only-pnpm.cjs.
• Points the compat workspace preinstall script at the root script.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
scripts/only-pnpm.cjs	Updates pnpm detection logic.
packages/test/test-version-utils/compat-workspaces/scripts/only-pnpm.cjs	Removes duplicated pnpm enforcement script.
packages/test/test-version-utils/compat-workspaces/full/package.json	Redirects compat workspace preinstall to the root pnpm enforcement script.