ci(infra): drop redundant opened trigger from issue-link check

[Mason Daugherty (mdrxy)]

Remove opened from the require_issue_link.yml trigger list. New PRs never have labels at creation time, so the job-level if (which requires the external label) always evaluated to skip — producing a noisy "Skipped" check on every internal PR. Enforcement chains through pr_labeler.yml → labeled event, making opened a no-op.

Changes

• Drop opened from pull_request_target.types in require_issue_link.yml — only edited, reopened, and labeled remain
• Update header comment to document why opened is intentionally excluded