refactor: mount plugins and config instead of baking into image

Image now contains only the derp package. Config and plugins are
bind-mounted at runtime, enabling live edits without rebuilds.

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

Containerfile

@@ -7,7 +7,4 @@ COPY src/ src/
 
 RUN pip install --no-cache-dir .
 
-COPY plugins/ plugins/
-COPY config/derp.toml.example config/derp.toml.example
-
 ENTRYPOINT ["derp"]