Add checkout compliance and run traceability to all agents

Add explicit POST /api/issues/{issueId}/checkout and PATCH status
update curl templates with X-Paperclip-Run-Id headers to all agent
heartbeats. Document Gemini workspace sandboxing in Hugh's TOOLS.md.
Also removed Regina's ghost instructionsFilePath from live DB.

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

engineering/regina/HEARTBEAT.md

@@ -24,12 +24,34 @@ Orient yourself:
 
 For each assigned issue:
 
+#### Checkout the issue first
+
+**You MUST checkout before doing any work. If you skip this, your work is untraceable.**
+
+    curl -sf -X POST "$PAPERCLIP_API_URL/api/issues/{issueId}/checkout" \
+      -H "Authorization: Bearer $PAPERCLIP_API_KEY" \
+      -H "Content-Type: application/json" \
+      -H "X-Paperclip-Run-Id: $PAPERCLIP_RUN_ID" \
+      -d '{"agentId": "8a627431-075d-4fc5-8f90-0bcac607e6ae", "expectedStatuses": ["todo", "backlog", "blocked"]}'
+
+Replace `{issueId}` with the actual issue ID. If checkout returns 409 (already claimed), skip to the next issue — never retry.
+
+#### Do the work
+
 - Read the full thread
 - Execute the requested testing or verification work
 - Document your findings clearly: what you tested, how, and what you found
-- Comment on the Paperclip issue with your results
 - If you found bugs, open GitHub issues on the affected repo with clear reproduction steps
-- Update issue status appropriately
+
+#### Update issue status
+
+**Every status change MUST include the X-Paperclip-Run-Id header.**
+
+    curl -sf -X PATCH "$PAPERCLIP_API_URL/api/issues/{issueId}" \
+      -H "Authorization: Bearer $PAPERCLIP_API_KEY" \
+      -H "Content-Type: application/json" \
+      -H "X-Paperclip-Run-Id: $PAPERCLIP_RUN_ID" \
+      -d '{"status": "done", "comment": "Describe what you tested, how, and what you found."}'
 
 ### 3. Review open PRs that need QA