deps: add scikit-learn — unblocks CI image publish (broken since plan 008)

The docker-image workflow's test gate has aborted at collection since sklearn
was introduced (test_conversion_probe.py, plan 008), so no image published.
Regime gate (plan 011) also needs sklearn at runtime in the image, else the
live gate silently fail-opens forever. Adds scikit-learn to requirements.txt
(installed by both the CI test job and the Dockerfile).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

requirements.txt

@@ -5,6 +5,7 @@ pytest>=7.0.0
 python-dotenv>=1.0.0
 pyyaml>=6.0
 numpy>=1.24.0
+scikit-learn>=1.5  # regime metalabel (plan 011) + conversion probe (plan 008)
 apscheduler>=3.10
 youtube-transcript-api>=0.6
 rich>=13.0