Instead of running Wiki Writer on closed PRs, run it once a day if any merges to default branch have happened

[mrjf]

https://github.com/githubnext/agentics/blob/b897c2f3e43bde9ff7923c8fa9211055b26e27cc/workflows/agentic-wiki-writer.md

Also document this on the doc page for it, suggesting that users can choose a different cadence or event structure if they like.