chore(gha): add yamllint.yml GitHub workflow

[ms-bot]

Why is this needed?

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

[Copilot]

Pull request overview

Adds a GitHub Actions workflow intended to validate YAML files in this repository, providing CI feedback on pushes/PRs and sending a notification on workflow failure.

Changes:

• Introduces a new .github/workflows/yamllint.yml workflow to run yamllint against the repo.
• Adds a follow-up notify job that runs when the validation job fails.

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[maximus-agents]

Generated workflow sync verified: CI is green and generated-workflow review threads were resolved.

[maximus-agents]

🤖 fix-approve-merge

Outcome: BLOCKED ⛔
Head commit: 467636f

Required Validate YAML files passes; the required Linear check fails before execution with Unable to resolve action mobsuccess-devops/github-actions-linear, not found. The current head is approved, squash auto-merge remains enabled, and all three Copilot threads are resolved with upstream-policy explanations; no generated workflow files were changed.

Next: Restore or grant access to mobsuccess-devops/github-actions-linear, rerun the Linear workflow, and verify the required check passes.