ci: auto-reopen external PRs after issue link requirement is satisfied (#35699)

Auto-reopen external PRs that were closed by the `require_issue_link`
workflow once the author fixes their PR description. Previously, the
workflow closed non-compliant PRs but required a maintainer to manually
reopen them — creating unnecessary back-and-forth when the contributor
just needed to add an issue link or get assigned.

## Changes
- Add reopen logic to the success path in `require_issue_link.yml`:
after removing the `missing-issue-link` label, call `pulls.update({
state: 'open' })` if the PR is closed *and* still carries the
`missing-issue-link` label — gating on the label ensures only
workflow-closed PRs are reopened, not PRs closed manually by maintainers
- Update the bot's auto-close comments to tell contributors the PR will
reopen automatically once they fix the issue, instead of directing them
to ask a maintainer