feat: add container support with Containerfile and make targets

- Containerfile: python:3.13-slim base, pip install, copy plugins
- .containerignore: exclude dev artifacts from build context
- Makefile: add build, container-run, container-stop, container-logs

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

Containerfile

@@ -0,0 +1,13 @@
+FROM python:3.13-slim
+
+WORKDIR /app
+
+COPY pyproject.toml .
+COPY src/ src/
+
+RUN pip install --no-cache-dir .
+
+COPY plugins/ plugins/
+COPY config/derp.toml.example config/derp.toml.example
+
+ENTRYPOINT ["derp"]