Add We are hiring news announcement

[fanyangCS]

Summary

• Add the dated We are hiring news item (2026-09-12), using the existing homepage/news-list → detail-page convention.
• Preserve all six paragraphs of Fan's approved English announcement verbatim.
• Include all three exact Microsoft Careers application URLs. All three public page titles identify the role as Researcher; labels use job IDs rather than unverified city mappings.
• Change only _news/we-are-hiring.md.

Verification

• Changed-file Prettier check and GitHub's full-repository Prettier check passed.
• PR production-mode Jekyll build passed, including Install and Build and CSS purge. Production deployment was correctly skipped for the pull_request event.
• ruby test/cache_bust_test.rb: 6 tests, 8 assertions, no failures.
• ruby test/repository_last_updated_test.rb: 2 tests, 3 assertions, no failures.
• Preview production-mode Jekyll build and deployment passed, including Install and Build, CSS purge, and preview publication.
• GitHub Pages build and deployment passed. An earlier Pages run was cancelled; this later successful run published the preview.
• Live HTTP/HTML verification passed: homepage, news index and detail page return HTTP 200; the first homepage news item is dated 2026-09-12; all six approved paragraphs match the source verbatim; all three exact application links are present in the requested order.
• The local full-site build reached its 180-second limit without reporting a build error; the successful CI production-mode build is the build proof. Browser visual verification was unavailable due to browser startup timeouts; live content/link verification used HTTP and parsed HTML.

Review and publication

Per the established homepage workflow, this PR is for Fan's review and merge. It does not publish the announcement to the production homepage until merged.

Verified live preview: https://fanyangcs.github.io/previews/hiring-news-2026-09-12/

Verified live preview detail: https://fanyangcs.github.io/previews/hiring-news-2026-09-12/news/we-are-hiring/

Production detail after merge/deployment: https://fanyangcs.github.io/news/we-are-hiring/

AI-generated implementation using user-approved copy; no substantive rewriting.