fix: exit 0 when no PRs need attention in Evergreen workflow

The pre-flight step was using sys.exit(1) for "no work to do" cases,
which marked the workflow as failed and triggered spurious failure
issues. Changed to sys.exit(0) for the normal "no open PRs" and
"no PRs need attention" paths. Added noop instruction to the agent
prompt so it handles the null selection gracefully.

Agent-Logs-Url: https://github.com/githubnext/tsessebe/sessions/4c6e25d0-358d-4732-b45a-4ed8a1e3ecb8

Co-authored-by: mrjf <180956+mrjf@users.noreply.github.com>

Author: Copilot