ci: use pull_request_target so auto-assign works on fork PRs

[modem7]

What

Switch the auto-assign-pr job from the pull_request trigger to pull_request_target.

Why

PRs opened from forks get a read-only GITHUB_TOKEN under pull_request, regardless of the job's permissions: block. The assign call is rejected, so the job fails on every fork PR, for example #160:

Resource not accessible by integration (403)
https://docs.github.com/rest/issues/assignees#add-assignees-to-an-issue

pull_request_target runs in the context of the base repo with a write-capable token, so assignment works for fork PRs too.

Safety

The usual pull_request_target risk is checking out and running untrusted PR code with a privileged token. This job never does that. It has no checkout step and only calls pozil/auto-assign-issue to assign modem7. Nothing from the PR is executed.

Notes

• The job's if: condition is updated to match the new event name.
• The github.actor != 'renovate[bot]' guard is unchanged.
• Same-repo PRs (for example Renovate) are unaffected.