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: AUTO-MERGE ENABLED ⏳
Head commit: fdc1902

Approved and enabled squash auto-merge after the branch was synchronized with master; YAML validation and Mobsuccess PR Compliance pass, Copilot review is complete with all threads resolved, but Linear and LinearMsTesters fail before execution because the private action mobsuccess-devops/github-actions-linear cannot be resolved.

Next: Restore GitHub Actions access/visibility for the private mobsuccess-devops/github-actions-linear action in the upstream policy/action project, then re-run the Linear workflows; the generated workflow in this PR must not be edited.