ci: add Slack notification workflow for teammate PRs to main

[esabou-snyk]

Summary

Adds .github/workflows/notify-slack-pr-review.yml, which posts a Slack message when a teammate opens (or marks ready for review) a non-draft PR targeting main.

• Trigger: pull_request with types: [opened, ready_for_review], branches: [main].
• Skips drafts and only fires for authors in the TEAMMATE_HANDLES repo variable (JSON array of GitHub usernames).
• Posts via slackapi/slack-github-action@v1.27.0 using an incoming webhook (SLACK_WEBHOOK_URL secret).
• Message includes PR title (as link), author handle, and a :memo: marker.

Setup required before merge

This PR is a draft because the workflow references config that must be added in the repo Settings UI first — otherwise every PR run would fail on fromJSON(vars.TEAMMATE_HANDLES):

1. Repo secret SLACK_WEBHOOK_URL — incoming webhook URL pointing at #team-docs.
2. Repo variable TEAMMATE_HANDLES — JSON array of teammate GitHub handles, e.g. ["alice","bob","charlie"].

Once both are set and this PR is marked ready, the guard clauses will start matching real events.

Test plan

• Configure SLACK_WEBHOOK_URL secret and TEAMMATE_HANDLES variable in repo Settings.
• Open a test PR from an allowlisted teammate against main → confirm one Slack message lands in #team-docs with correct title, link, and author.
• Open a draft PR from an allowlisted teammate → no Slack message. Mark it ready for review → exactly one message fires.
• Open a PR from a handle not in TEAMMATE_HANDLES → workflow runs but notify job is skipped (visible in the Actions tab).
• Open a PR against a non-main base → workflow does not trigger.

🤖 Generated with Claude Code

[snyk-io]

✅ Snyk checks have passed. No issues have been found so far.

Status	Scan Engine	Critical	High	Medium	Low	Total (0)
✅	Open Source Security	0	0	0	0	0 issues
✅	Licenses	0	0	0	0	0 issues
✅	Code Security	0	0	0	0	0 issues

💻 Catch issues earlier using the plugins for VS Code, JetBrains IDEs, Visual Studio, and Eclipse.