docs: add clean Devin integration docs

Author: samfert

.github/DEVIN.md

@@ -0,0 +1,22 @@
+# Devin CI Auto-Fix
+
+Automatic CI failure fixing powered by Devin AI.
+
+## How it works
+
+When a CI build fails on a **feature branch**, Devin automatically:
+1. Receives the failing branch name
+2. Clones the repo and checks out that branch
+3. Runs the build to see what failed
+4. Fixes the issue
+5. Pushes the corrected code
+
+## Requirements
+
+- `DEVIN_API_KEY` secret configured in repository settings
+- Get your API key from https://app.devin.ai/settings/api-keys
+
+## Workflow
+
+- `.github/workflows/devin-ci-fix.yml` - Triggers Devin on feature branch failures
+- Only runs on feature branches (not master or devin/* branches)