ci: fix the lint findings that stop CI before any test runs

[nk0952]

Summary

CI on master is red, and it fails at the Lint (ruff) step in all nine matrix legs. Everything after that step - yamllint, mypy, Run test suite - is skipped. So the red X isn't a failing test being reported; CI hasn't got as far as running one.

That has been hiding a real regression. python -m pytest tests/ fails 9 tests in tests/unit/test_index_sync.py on master right now, and CI has never mentioned it.

This clears the lint and type findings sitting between the workflow and its own test step.

Type of Change

• ci - CI/CD pipeline changes

Changes

ruff check . reports four findings on master, all in test files:

• tests/ebuild/test_build_dir_resolution.py:31 - F811, import shutil appears twice
• tests/unit/test_ci_gate.py:214,215 - E402, two imports sitting below module-level code
• tests/ebuild/test_package_recipe.py:117 - W292, no newline at end of file

Fixing those gets the job as far as Type check (mypy), which then fails on ebuild/plugins/__init__.py:46:

error: Argument 2 to "get" of "Deprecated" has incompatible type "list[Never]"; expected "EntryPoints"  [arg-type]
note: Error code "arg-type" not covered by "type: ignore[attr-defined]" comment

The # type: ignore there names the wrong error code, so it was never silencing anything. I spelled out the pre-3.10 entry_points() mapping shape with cast rather than widening the ignore. That branch only runs on Python 3.8/3.9, which the matrix doesn't cover, so behaviour is unchanged either way.

Testing

Python 3.11.15, pip install -e ".[dev]", ruff 0.16.6. I ran each CI step by hand with the same flags ci.yml uses:

step	master	this branch	+ #119
ruff check .	4 errors	pass	pass
yamllint .	pass	pass	pass
mypy ...	2 errors	1 error	pass (107 files)
pytest tests/	9 failed, 669 passed	9 failed, 669 passed	678 passed, 0 failed

• All existing tests pass - 669 before, 669 after, no expectations changed
• Manual testing performed - each workflow step run individually

Nothing outside the cast touches a shipped code path.

Related Issues

The 9 remaining failures are test_index_sync.py, which open PR #119 fixes. I've left those alone rather than duplicate that work - landing both gives a green gate, which is the last column above.

Additional Notes

Two things I noticed but didn't fold in, since they look like maintainer calls:

1. ci.yml installs its linters unpinned. A new ruff release can turn the gate red with no change to this repo, which may well be how this arrived. Pinning them, or moving them into the dev extra in pyproject.toml, would make the gate reproducible.
2. .coveragerc sets fail_under = 100 against a suite measuring ~24%, and ci.yml works around it with --cov-fail-under=0 plus a comment calling it a maintainer decision. Probably worth settling in one place.