Install Evergreen workflow to fix CI triggering and PR health

[mrjf]

What

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

Why

Autoloop PRs have no CI running because safe-outputs pushes via GITHUB_TOKEN, which doesn't fire pull_request events. The agent wastes ~15 turns per iteration polling for CI checks that never appear, burning millions of tokens.

Evergreen solves this by:

• Detecting autoloop PRs with no check runs and triggering CI (via empty commit push using GH_AW_CI_TRIGGER_TOKEN)
• Merging main into PRs that are behind
• Fixing failing CI (type errors, lint, test failures)

Steps

1. Copy .github/workflows/evergreen.md from tsessebe
2. Adapt for apm:
   • Network allowed list: add go ecosystem
   • Reference this repo's ci.yml for CI triggering
3. Compile: gh aw compile evergreen
4. Verify GH_AW_CI_TRIGGER_TOKEN secret is set (it is: 2026-05-13)

Prerequisites

• GH_AW_CI_TRIGGER_TOKEN secret set
• ci.yml exists with pull_request trigger
• ci.yml needs workflow_dispatch trigger added (for manual reruns)