ci: auto-close external PRs failing `issue-link` check (#35697)

Auto-close external PRs that fail the issue-link or assignee check
instead of just failing the CI status. The bot comment now explains the
PR was closed and gives numbered steps to resolve — including asking a
maintainer to reopen, since external contributors can't reopen PRs
themselves.

## Changes
- Close the PR via `pulls.update` after posting the bot comment in the
`check-issue-link` job, gated on `state === 'open'` to avoid redundant
API calls on re-runs
- Rewrite bot comment copy for both failure modes (missing link, not
assigned) to lead with "This PR has been automatically closed" and end
with "ask a maintainer to reopen this PR"