ci: skip auto-assign for bot-authored pull requests

GitHub cannot assign bot accounts, so every Dependabot PR made the
Auto Assign PR workflow POST {"assignees":["dependabot[bot]"]} and
receive 403 Forbidden, leaving a failing check unrelated to the change
under review.

Guard the job on the author type so it runs only for human authors, and
grant `issues: write` — addAssignees belongs to the issues API and was
not covered by the existing `pull-requests: write` grant.

Author: rashmiadvani

.github/workflows/auto-assign.yml

@@ -5,10 +5,14 @@ on:
     types: [opened]
 
 permissions:
+  issues: write
   pull-requests: write
 
 jobs:
   assign:
+    # Bot accounts (dependabot[bot] and friends) cannot be assigned; the API
+    # rejects them with 403, which failed this workflow on every automated PR.
+    if: github.event.pull_request.user.type != 'Bot'
     runs-on: ubuntu-latest
     steps:
       - name: Assign PR to author