Fix empty RUN_COUNT causing invalid jq expression in agent-deep-dive workflow (#1289)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: Copilot

.github/workflows/agent-deep-dive.lock.yml

@@ -71,6 +71,7 @@ steps:
     run: |
       set -euo pipefail
       mkdir -p /tmp/gh-aw/deep-dive
+      RUN_COUNT="${RUN_COUNT:-20}"
 
       # If no target workflow is specified, pick one automatically by rotating
       # through agentic workflows based on the current day-of-week