build: keep the image context clean instead of wiping the worktree

Every test target required 'clean', which is 'git clean -fdX .' - so running the unit tests deleted every git-ignored file the operator had, venv and caches included. It was compensating for a missing .dockerignore: the Dockerfile's COPY . . otherwise drags __pycache__, egg-info and build output into the image context.

The ignore file fixes that where it belongs, so the prerequisite can go. 'clean' remains available as its own target.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,13 @@
+.git
+.github
+__pycache__
+**/__pycache__
+*.egg-info
+**/*.egg-info
+artifacts/
+dist/
+build/
+.venv
+.ruff_cache
+.pytest_cache
+.mcp.json