Add a news section written as Markdown in the repository

[openipc-ai]

Part of #206 (Phase 1). Depends on nothing.

Why

June's navigation has seven entries and the seventh is News. It was deferred to Phase 2 along with its mechanism: posts as Markdown files in the repository, rendered with kramdown, so an announcement is a pull request and not a database row. Nothing of this exists. There is no Markdown renderer in the Gemfile, package.json or app/, no /news route, and the navbar has six entries.

What to change

• kramdown in the Gemfile. Posts live in content/news/<YYYY-MM-DD>-<slug>.md with a small front matter block (title, date, summary, author); a NewsPost PORO reads the directory, parses the front matter, renders the body, and sorts by date. Loaded once per process in production, per request in development.
• /news (index: title, date, summary) and /news/<slug> (the post), full-bleed, plus /news.atom from a Rails builder template so readers and the Telegram channel can subscribe.
• Internal links inside a post go through the same locale_path rewriting that app/helpers/application_helper.rb:91-107 applies to translated copy, so /supported-hardware in a post lands in the reader's language.
• English only at launch, as June decided; the index and posts render under every prefix without a missing-translation string, and the i18n gate is told so.
• News becomes the seventh navbar entry and a footer link. The sitemap lists the index and each post.
• One real first post, so the section does not launch empty. Candidate: the relaunch itself.
• A test renders every file in content/news/ and fails on malformed front matter or an unsafe HTML element, so a bad post breaks CI rather than a request.

Done means

• /news, one post and /news.atom render; the feed validates.
• A post with broken front matter fails bin/rails test and does not reach production.
• Screenshots of the index and a post before any PR.