fix: new workers now check in more reliably; fix: more `skipped` keys to 0 instead of null

[tazlin]

• The worker check-in fix would come up very rarely in production but was presenting a challenge for local development and for the CI runner.
• It was incidentally noted that certain keys were unset (and unsent) from the skipped dict when they didn't apply and for consistency and completeness we're going to initialize them to 0 instead.