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>
