chore: fill in Pixel Patty agent ID and GitHub App credentials

Replace placeholders in CONFIG.md and HEARTBEAT.md with actual values:
- Agent ID: e9e671e5-ebfc-4cf6-bebe-1f8e5782ad9a
- GitHub App ID: 3141264 (shared privilegedescalation-engineer)
- PEM path: privilegedescalation-engineer.pem

Co-Authored-By: Paperclip <noreply@paperclip.ing>

engineering/patty/HEARTBEAT.md

@@ -29,7 +29,7 @@ For each assigned issue:
       -H "Authorization: Bearer $PAPERCLIP_API_KEY" \
       -H "Content-Type: application/json" \
       -H "X-Paperclip-Run-Id: $PAPERCLIP_RUN_ID" \
-      -d '{"agentId": "<AGENT_ID_PLACEHOLDER>", "expectedStatuses": ["todo", "backlog", "blocked"]}'
+      -d '{"agentId": "e9e671e5-ebfc-4cf6-bebe-1f8e5782ad9a", "expectedStatuses": ["todo", "backlog", "blocked"]}'
 
 Replace `{issueId}` with the actual issue ID. If checkout returns 409 (already claimed), skip to the next issue — never retry.