test: fold skill-discovery check into idempotency test
test_skill_discovery_from_journal_cwd asserted the existence of
journal/.claude/skills/journal/SKILL.md at the repo root — a path
that's gitignored, produced only by `make skills`, and not a
make-test prerequisite. Fresh-clone runs hit it before `make
install` had built the artifact and failed deterministically.
Move the assertion into test_make_skills_idempotent's tmp tree:
after `make skills` runs, verify the symlinked SKILL.md resolves
and its content starts with frontmatter. Same property, hermetic
setup, no dependency on dev-box state.