CI consolidates onto GitHub (primary remote); Gitea (secondary) has Actions disabled. Move the Linux pipeline — unit + widget + golden + a11y + coverage gate, integration_test (xvfb), bundle smoke, dart doc — verbatim from .gitea/workflows/test.yml (its own header said to copy it here on GitHub), and delete the dead Gitea copy. Joins windows.yml + release.yml. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>