add: /contributions page (EN+FR) + link from homepage OSS brick

New page listing OSS PRs shipped upstream, with insight summary, tags,
and links (PR / issue / blog pr-note). Includes a 'what's cooking' section
for watched projects (changedetection watchers piping to ntfy).

The homepage OSS brick now has a third link pointing to /contributions
for the full shelf view.

Philosophy: PRs earn an article by insight size, not diff size.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ferr079