CI for triggering a delete of the ns

### Describe the bug
I had to manually delete some ns of past PRs that were closed, for example #765, #727, even though they were merged/or manually closed. While this is not a super concerning, I would like to keep our resources running at a minimum to avoid incurring more charges.

### To Reproduce
Not sure if it's reproducible, I didn't want to close any PRs. We can try opening fake PR and triggering the pr on event workflow?

### Logs
n/a

### Screenshots
n/a

### Environment
n/a

### Additional context
Whoever investigates this can reach out to me!

### Exposure
n/a

Inspect the PR event workflow and the existing namespace lifecycle for preview environments, then use a disposable test PR to reproduce the cleanup gap. Done means namespaces for merged or manually closed PRs are deleted automatically and the workflow can be verified without leaving billable resources behind.

Written by the indexing model from the issue text.