ci(infra): require issue link for external PRs (#35690)

Enforce that all external PRs reference an approved issue via GitHub
auto-close keywords (`Fixes #NNN`, `Closes #NNN`, `Resolves #NNN`). This
replaces the previous AI-disclaimer policy in the PR template with a
stricter requirement: external contributors must link to a
maintainer-approved issue before their PR can merge.

## Changes

- Add `require_issue_link.yml` workflow that chains off the `external`
label applied by `tag-external-contributions.yml` — listens for
`labeled`, `edited`, and `reopened` events to avoid duplicating the org
membership API call
- Scan PR body with a case-insensitive regex matching all conjugations
of `close/fix/resolve` + `#NNN`; fail the check and post a deduplicated
comment (via `<!-- require-issue-link -->` HTML marker) when no link is
found
- Apply a `missing-issue-link` label on failure, remove it on pass —
enables bulk cleanup via label filter
- Add `workflow_dispatch` backfill job to `pr_size_labeler.yml` for
retroactively applying size labels to open PRs
- Quote `author` in GitHub search queries in
`tag-external-contributions.yml` to prevent mismatches on usernames with
special characters
- Update `PULL_REQUEST_TEMPLATE.md` to replace the AI-disclaimer
guideline with the new issue-link requirement

> [!NOTE]
> `require_issue_link.yml` depends on `tag-external-contributions.yml`
running first to apply the `external` label. Deploy as a non-required
check initially, then promote to required after validation.