ci: parallelize tests with pytest-xdist and close CI gaps

[yarikoptic]

ci: parallelize tests with pytest-xdist and close CI coverage gaps

Two orthogonal improvements bundled for one round trip:

1. Parallelize pytest via pytest-xdist

   • Add pytest-xdist>=3.0 to the test extras.
   • Default tox [testenv] command to
     pytest -n 6 --dist loadscope tests/, still overridable via
     posargs (tox -e py312 -- tests/test_cli.py,
     tox -e py312 -- -n 0 tests/test_migrate.py::TestX both work).
   • loadscope chosen after measuring: it groups by class/module so
     parametrized integration sweeps and the _register_synthetic_2x_rules
     class-scoped fixture stay on one worker. loadfile was ~2x slower
     on the current suite.
   • Hazard audit: only monkeypatch.chdir is used (per-process safe);
     _RULES mutation in _register_synthetic_2x_rules is class-local
     and no test outside TestMigrate20 reads those synthetic rules, so
     xdist process isolation is fine.

2. Close CI coverage gaps asserted in PR Initial design + implementation (round 2): migration engine, annex correctness, dedup #7's test plan

   • Add "3.14" to the workflow matrix.
   • Add duplication to the 3.12 [gh-actions] entry so pylint
     duplicate-code actually runs in CI (previously only listed in
     envlist locally).
   • Install git-annex via apt before the tox run so the
     @requires_git_annex integration tests exercise both plain-git and
     git-annex paths.

Co-Authored-By: Claude Code 2.1.116 / Claude Opus 4.7 (1M context) noreply@anthropic.com