feat: switch CI to linux runner with podman containers

Replace ubuntu-latest runner with linux label and migrate all
container operations from docker to podman. Add requirements.txt
as single source of truth for runtime dependencies.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Containerfile

@@ -1,10 +1,11 @@
 FROM python:3.13-alpine
 
-RUN pip install --no-cache-dir --upgrade pip && \
-    pip install --no-cache-dir pyyaml>=6.0
-
 WORKDIR /app
 
+COPY requirements.txt .
+RUN pip install --no-cache-dir --upgrade pip && \
+    pip install --no-cache-dir -r requirements.txt
+
 ENV PYTHONUNBUFFERED=1 \
     PYTHONDONTWRITEBYTECODE=1 \
     PYTHONPATH=/app/src