ci: add guardrails workflow [skip deploy]

[cchawn]

Warning

This PR was opened automatically Please review it carefully for changes Reach out to #developer-tools on Slack if you have any questions!

https://wealthsimple.atlassian.net/browse/BEPLAT-1239

Why

Developer Tools is rolling out a new "Guardrails" workflow across all repositories.

This workflow calls a shared workflow on PR open, edit, or synchronize. The workflow includes essential language-agnostic checks to enforce best practices. At the moment, this includes checks for:

1. Any Slack URLs in the code (currently enabled)
2. A Jira issue referenced in the PR title/body or branch name – read the announcement for more context (currently disabled)

Important

The new Guardrails check is currently not required to merge PRs!

What Changed

• Add a new guardrails.yml GitHub workflow

[github-actions]

Pull reviewers stats

Stats of the last 30 days for odef:

	User	Total reviews	Time to review	Total comments

^{⚡️ Pull request stats}

[lvets]

This PR refers to private Wealthsimple repositories wealthsimple/github-workflows & private ticketing systems (https://wealthsimple.atlassian.net/browse/BEPLAT-1239) and should not have been opened in this public repository.

Same goes for the same PRs opened in the other public repos.

@cchawn @aviyachki