Add FAQ last_modified workflow

[lyzhang0113]

Summary

• Add a GitHub Actions workflow that updates last_modified for FAQ markdown files changed in PRs targeting master.
• Keep the workflow scoped to same-repo PRs so it can safely push timestamp commits back to the PR branch.
• Add concurrency, explicit permissions, UTC timestamp formatting, and a rebase before pushing to reduce workflow noise and push failures.

Test Plan

• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/update-last-modified.yml"); puts "YAML OK"'
• xxd -l 8 .github/workflows/update-last-modified.yml

[Copilot]

Pull request overview

Adds an automated GitHub Actions workflow to keep last_modified front matter up to date for FAQ markdown files when they’re changed in pull requests targeting master, with safeguards to only run on same-repo PRs so it can push a timestamp commit back to the PR branch.

Changes:

• Introduces a new pull_request-triggered workflow scoped to _articles/faq/*.md changes.
• Detects changed FAQ files vs the PR base branch, updates/creates last_modified in YAML front matter, and commits + pushes back to the PR branch.
• Adds concurrency control and explicit token permissions to reduce races and push failures.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.