Propagate shapepipe_run's exit code (main must return run's value)

`main()` called `run(args)` but discarded its return value, so `exit(main())`
was always `exit(None)` → 0. `run()` returns 1 when it catches an error
(`catch_error` + `return 1`), so *every* handled failure has been exiting 0 —
invisible to `exit $?` in the job scripts and to any CI/automation. One-word
fix: `return run(args)`. Add a regression test that main forwards run's value.

Surfaced while end-to-end testing the MPI singleton guard: the guard fired and
logged loudly but the job still exited 0 until this fix.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: cailmdaley

src/shapepipe/shapepipe_run.py

@@ -15,7 +15,7 @@
 
 def main(args=None):
 
-    run(args)
+    return run(args)
 
 
 if __name__ == "__main__":

tests/unit/test_entrypoints.py

@@ -47,3 +47,18 @@ def test_console_entrypoint_help_runs(entrypoint):
 
     assert result.returncode == 0, result.stderr
     assert "usage:" in result.stdout.lower()
+
+
+@pytest.mark.parametrize("exit_code", [1, None])
+def test_main_propagates_run_exit_code(monkeypatch, exit_code):
+    """``main`` must forward ``run``'s return value.
+
+    ``run`` returns 1 when it catches an error; if ``main`` drops that,
+    ``exit(main())`` becomes ``exit(0)`` and every handled failure looks like
+    success to the batch system.
+    """
+    import shapepipe.shapepipe_run as entry
+
+    monkeypatch.setattr(entry, "run", lambda args=None: exit_code)
+
+    assert entry.main() == exit_code