Add __main__.py entry point to enable CLI execution via python -m openjarvis.cli.

Previously, running `uv run jarvis start` would fail to start the process with error:
"No module named openjarvis.cli.__main__; 'openjarvis.cli' is a package and cannot be
directly executed" because the CLI package lacked an entry point.

Signed-off-by: Patrick Fung <patrick_fung@foxmail.com>

src/openjarvis/cli/__main__.py

@@ -0,0 +1,6 @@
+"""Entry point for CLI execution."""
+
+from openjarvis.cli import main
+
+if __name__ == "__main__":
+    main()