chore(gha): add ms-testers.yml GitHub workflow

[ms-bot]

Why is this needed?

This pull request has been created by a robot to add the ms-testers.yml GitHub workflow.

[Copilot]

Pull request overview

Adds a new GitHub Actions workflow intended to sync “ms-testers” assignee information to Linear on merge queue and pull request activity.

Changes:

• Introduces .github/workflows/ms-testers.yml workflow with merge_group and pull_request triggers
• Adds a single job that runs a Linear integration action with LINEAR_API_KEY and GITHUB_TOKEN

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[maximus-agents]

🤖 fix-approve-merge

Outcome: BLOCKED ⛔
Head commit: fdc1902

Approved PR #9 remains open with squash auto-merge enabled; YAML and Mobsuccess Compliance pass, but Linear and LinearMsTesters fail before execution because the private github-actions-linear action is not accessible, and no Linear ticket ID is present in the PR title.

Next: An organization administrator must grant the private action organization access (upstream github-mobsuccess-policy remains the source of the generated workflows), then re-run both Linear workflows; do not edit the generated PR.