Add YAML lint step to CI pipeline

[johnmccrae]

Context

During ex10, the azure-pipelines.yml had silently concatenated lines (missing newlines) that caused "no pipelines found" in Azure DevOps — only caught because the UI was broken, not by any automated check.

Acceptance Criteria

• A YAML lint step (e.g. yamllint or powershell -Command [yaml]::parse) runs on every PR
• Step is placed before all build tasks so a malformed YAML file fails fast
• Step is continueOnError: false (this one should be blocking)
• Documented in CONTRIBUTING.md

Relevant Code Paths

• azure-pipelines.yml — the file that broke
• .github/pull_request_template.md — add a reminder to validate YAML before push