Install Evergreen workflow from tsessebe

[mrjf]

What

Install the Evergreen PR health keeper workflow, adapted from githubnext/tsessebe.

Evergreen runs every 5 minutes and automatically:

• Merges main into PRs that are behind
• Fixes failing CI (type errors, lint, test failures)
• Triggers CI on autoloop PRs that have no checks running

Steps

1. Copy .github/workflows/evergreen.md from tsessebe
2. Adapt for tsikit-learn:
   • Network allowed list: keep node, add python if needed
   • Ensure GH_AW_CI_TRIGGER_TOKEN secret is referenced
   • Update the CI trigger function to reference this repo's ci.yml
3. Compile: gh aw compile evergreen
4. Set GH_AW_CI_TRIGGER_TOKEN secret if not already set
5. Verify first run picks up any open PRs with failing CI

Prerequisites

• GH_AW_CI_TRIGGER_TOKEN secret set on the repo
• ci.yml has workflow_dispatch trigger (check — may already have it)