Scheduled items do not continue to show up in `org-agenda`

### What did you expect to happen?

When `org-agenda-span` is set to `day`, scheduled items should continue to how up in `org-agenda`, even after their scheduled date.

### What actually happened?

When `org-agenda-span` is set to `day`, scheduled items do not show up in `org-agenda` after their scheduled date.

### Describe your attempts to resolve the issue

The reason is that `org-agenda-start-day` is set to "-3d", i.e., a negative value, in the Doom configuration. This means that when the `org-agenda-span` does not cover three days in the past, scheduled items are hidden. In my optinion, this setting should only be set if `org-agenda-span` is covering "-3d". This is the case by default, but as soon as the user changes this setting, unexpected things are happening.

Any thoughts?

### Steps to reproduce

Put in user config:
```
(setq org-agenda-span 'day)
```

To solve the issue, add to user config:
```
(setq org-agenda-start-day nil)
```

### System Information

https://pastebin.com/USs8paG6

Reproduce the issue from the org-agenda entry point with org-agenda-span set to 'day', comparing the default org-agenda-start-day with nil. Trace where Doom sets org-agenda-start-day and verify that scheduled items remain visible after their scheduled date when the agenda spans one day.

Written by the indexing model from the issue text.