Fix daily PR summary cron script

Rewrite bin/generate-pr-summary.sh to be a pure stdout formatter (no tool calls). The cron session runs it, checks check-dm-quiet.sh, and sends via the message tool only if output is non-empty. Mark existing fleet-task 64e897af-76e2-49f6-a366-3b035c4f4e10 completed after verifying output.