vf-holdout

Holdout texts are kept away from all learning, so tests can measure whether the skill writes like the user.

1. add <file> [profile]: run, from this skill's folder, python3 ../voice-forge/scripts/vf.py holdout add <file> [--profile <profile>]. It copies the file into holdout/ (never overwriting) and saves it to the private repo.
2. list: run python3 ../voice-forge/scripts/vf.py holdout list. Show the file names only.
3. Do not open, read, summarize or quote holdout texts. Do not use them in training, drafting, editing or analysis. Only a test run the user starts may read them.
4. Good holdout texts: the user wrote them alone, they are recent, and no round, sample or analysis uses them. Include one text the user considers already good.