docker-talkies v0.15.3 — docs catch up with the chatterbox executor

docs/models.md listed ten of the eleven executors in VALID_EXECUTORS, omitting
chatterbox, and the agent skill still described the pre-Chatterbox TTS surface.
Both now match the source, and the release script derives the check from
config.py and models.json rather than trusting the prose.

No code changed.