Sitemaps: Generate Daily Sitemap Content

### Description

When a user or search engine accesses a daily sitemap for a post type (e.g., `/sitemap-posts-2026-05-26.xml`), the site should respond with a well-formed sitemap for the posts that were published on that day, including their last modified date and time.

A query that yields post IDs and last modified dates for posts is:

```sql
SELECT ID, post_modified
FROM wp_posts
WHERE post_type = 'post'
AND post_status = 'publish'
AND post_date >= '2025-10-11'
AND post_date < '2025-10-12'
ORDER BY post_date DESC
```

The sitemap will need to include the permalink to the post as well as the last modified date.

### Use Case

When a user or search engine spider accesses the daily sitemap, they should be presented with a list of URLs with last modified dates matching the given post type and date.