ci: grant issues:write to auto-assign-pr job

[modem7]

Summary

• auto-assign-pr only declared pull-requests: write, but pozil/auto-assign-issue reads/writes assignees via the /issues/{number} API (PRs are issues under the hood), which needs issues scope
• GitHub granted this implicitly and inconsistently depending on the repo — some runs failed with Resource not accessible by integration
• Adds explicit issues: write, matching the sibling auto-assign-issue job

Test plan

• Confirm the auto-assign-pr job succeeds on this PR