installSkills() was copying SKILL.md files to both project-level
(.claude/skills/ in tmpDir) and user-level (~/.claude/skills/).
Writing to the user's real install fails when symlinks point to
different worktrees or dangling targets (ENOENT on copyFileSync).
Now installs to project-level only. The test already sets cwd to
the tmpDir, so project-level discovery works.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>