Maintainer sweep: assign a PR to the assignee of the issue it closes #1889

Problem

The maintainer sweep (.github/workflows/maintainer-sweep.yml) assigns PRs and issues separately. routePR routes by changed files and routeIssue routes by title/body text, each with its own load count. A PR that closes an issue therefore usually gets a different assignee than that issue. On 2026-10-23 all 4 open closing pairs were mismatched (#1791/#1788, #1705/#1703, #1690/#1689, #1559/#1558); those were reassigned by hand.

collect.ts already reads closingIssuesReferences into pr.linkedIssues, but routing does not use it.

Change

1. In scripts/maintainer/sweep.ts planSweep: when an unassigned PR closes an issue that has an assignee, and that assignee is a roster maintainer who did not author the PR, assign the PR to them. This wins over the core-review (AlemTuzlak) rule. Otherwise route as today.
2. Add a test in scripts/maintainer/sweep.test.ts covering: issue assignee wins (incl. over core-review), author fallback to normal routing.
3. Update the "Assignment order" section of scripts/maintainer/README.md.
4. Run the sweep with plain node instead of tsx (package.json maintainer:sweep, and any other tsx script in scripts/maintainer). Drop tsx from devDependencies if nothing else uses it.