Add scheduled blog post publishing

[madolson]

Summary

Add a GitHub Actions workflow that automatically publishes blog posts on a scheduled date.

Approach

• Add a publish_date field to blog post front matter (posts with a future date are excluded from the build)
• Create a scheduled workflow (e.g. daily cron) that checks if any unpublished posts have reached their publish date
• If new posts are ready, trigger a site rebuild/deploy

This lets authors merge blog posts ahead of time and have them go live on a specific date without manual intervention.

Open questions

• What cadence? Daily is probably fine, hourly if we want tighter control
• Should the workflow commit a change (e.g. flip a draft: true flag) or just trigger a rebuild that respects the date?

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