Add HOME env to all agents to isolate gitconfig and gh auth state

Each agent gets HOME set to their cwd so ~/.gitconfig and
~/.config/gh/ don't collide between concurrent heartbeats.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

engineering/regina/CONFIG.md

@@ -33,6 +33,7 @@
 {
   "cwd": "/paperclip/privilegedescalation/engineering/regina",
   "env": {
+    "HOME": { "type": "plain", "value": "/paperclip/privilegedescalation/engineering/regina" },
     "OPENROUTER_API_KEY": { "type": "plain", "value": "<REDACTED - restore from pg-fix-regina-env2.sh>" },
     "GITHUB_APP_ID_REGINA": { "type": "plain", "value": "3033788" },
     "GITHUB_PEM_PATH_REGINA": { "type": "plain", "value": "/paperclip/secrets/github-pems/regina.pem" }