tests: stop copying live journal in make-skills idempotency test
shutil.copytree(repo_root / "journal", ...) was the hidden quadratic
behind the test_make_skills_idempotent timeout flake — it copied the
dev's gitignored working journal (144GB / 466k files on the dev box,
chronicle/ alone was 135GB). 30s budget burned in os.sendfile before
make skills ran. make skills only needs an empty journal/ to write
.agents/skills and .claude/skills symlinks into.
Test runtime: 30s+ timeout failure -> 0.22s. Suite: 223s with 2
failures -> 174s green (req_be2hxxsh).