Strip the dependency detector and TMG test split out of the CI PR

Per the maintainer's earlier review ("too many completely different changes
in a single commit"), this PR should carry only the CI infrastructure
(workflows, image, the tests/ harness, run_addon_tests.py /
addon_system_deps.py / gi_bootstrap). Two standalone pieces that had
re-grown here move to their own follow-up PRs:

- tests/test_addon_dependencies.py — the undeclared-depends_on dependency
  detector (Mantis-13707 class), a standalone feature; ships in its own PR
  with its per-module isolated-load loop parallelised.
- the TMGimporter test rename (test_libtmg.py -> test_linux_libtmg.py) — a
  per-addon test edit; ships in its own PR as the test_<os>_* convention
  demonstrator.

This commit removes both from #820, leaving only the CI infrastructure.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: eduralph