Exempt verified Dependabot updates from the issue-closer gate #2480

Problem

The Closing keyword to open issue check fails automated dependency updates that do not have a canonical issue. On PR #2462, this is the only failed check on the current head. The repository reconciliation workflow already excludes Dependabot dependency bumps from missing-issue drift, while the issue-closer workflow still rejects them. Creating a separate issue for each automated version bump would add bookkeeping without improving review.

Completion criteria

• Exempt only PRs whose event author is dependabot[bot], whose head repository is this repository, and whose head branch begins dependabot/.
• Keep the existing open-issue closing-keyword check for human PRs, forks, and mismatched branches.
• Test the gate with both accepted and rejected cases, then verify the check on a real Dependabot PR after the workflow fix reaches main.
• Keep security, dependency review, and other CI checks in force.