Add comment for why pull_request_target is needed

[surchs]

https://github.com/neurobagel/workflows/blob/656439b0ecbfb0571f25c3c7ae14b62004bacb39/template_workflows/project_automation/handle_external_pr.yml#L4

This line has been a hard won victory over GH workflows particular handling of GITHUB_TOKEN: https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target we should include this in a comment so we don't forget

[github-actions[bot]]

We want to keep our issues up to date and active. This issue hasn't seen any activity in the last 75 days. We have applied the _flag:stale label to indicate that this issue should be reviewed again. When you review, please reread the spec and then apply one of these three options:

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