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>

.containerignore

@@ -0,0 +1,13 @@
+.venv/
+.git/
+.ruff_cache/
+.pytest_cache/
+.mypy_cache/
+__pycache__/
+*.pyc
+*.egg-info/
+build/
+dist/
+tests/
+docs/
+config/derp.toml