fix: add .dockerignore to prevent console script corruption (fixes #23)

Build artifacts (__pycache__, *.egg-info, dist/, etc.) were leaking into
the Docker build context via COPY, corrupting the pip-generated console
script's shebang line and causing bash to interpret it as a shell script.

- Add .dockerignore to exclude build artifacts and dev files
- Add __main__.py for python -m open_terminal support

.dockerignore

@@ -0,0 +1,11 @@
+__pycache__
+*.egg-info
+dist
+build
+.git
+.gitignore
+.github
+*.pyc
+.env
+.venv
+venv

open_terminal/__main__.py

@@ -0,0 +1,3 @@
+from open_terminal.cli import main
+
+main()