cli: add __main__ guard so python -m cmos.cli actually runs

Without `if __name__ == "__main__": sys.exit(main())` at the bottom
of cli.py, `python -m cmos.cli format <path>` imports the module
but never invokes main(), so the process silently exits 0 with
empty stdout — indistinguishable from a successful run that
produced no output. Discovered during real-draft testing on
2026-04-11.

Adds a regression test that subprocesses the CLI with --help and
asserts on stdout content. argparse --help exits 0 in both broken
and fixed states; stdout content is the only discriminator.

Both invocation paths now work:
- uv run cmos format <path>                (pyproject script entry)
- uv run python -m cmos.cli format <path>  (module invocation)

tests/test_cli.py

@@ -5,6 +5,8 @@ formatter so no API calls happen; the goal is to pin the glue logic, not the
 LLM behavior.
 """
 
+import subprocess
+import sys
 from pathlib import Path
 
 import pytest
@@ -68,6 +70,37 @@ def test_reformat_draft_bibliography_heading_preserved():
     assert "## Bibliography" in output
 
 
+def test_python_dash_m_invocation_actually_runs_main():
+    """Regression test: `python -m cmos.cli` must actually invoke main().
+
+    Without an `if __name__ == "__main__"` guard at the bottom of cli.py,
+    `python -m cmos.cli` imports the module body but never calls main(),
+    so the process silently exits 0 with empty stdout. That looks
+    indistinguishable from a successful run that produced no output —
+    the worst kind of bug, since callers assume the pipeline ran. This
+    test forces the guard to exist by invoking the CLI as a subprocess
+    with --help and asserting argparse actually fired.
+    """
+    result = subprocess.run(
+        [sys.executable, "-m", "cmos.cli", "--help"],
+        capture_output=True,
+        text=True,
+        timeout=10,
+    )
+    # argparse --help exits 0 whether or not main() ran, so the
+    # discriminator is the stdout content. Without main(), stdout is empty.
+    assert result.returncode == 0, (
+        f"expected exit 0, got {result.returncode}; stderr={result.stderr!r}"
+    )
+    assert result.stdout, (
+        "stdout was empty — `python -m cmos.cli` likely silently exited "
+        "without invoking main(). Check that cli.py has an "
+        '`if __name__ == "__main__": sys.exit(main())` guard at the bottom.'
+    )
+    assert "format" in result.stdout
+    assert "usage" in result.stdout.lower()
+
+
 def test_reformat_draft_preserves_order_under_concurrency():
     # With concurrent execution the formatter is called on all entries in
     # parallel; the CLI must reassemble them in input order regardless of