fix(issue-36): merge review - a prompt file the runner cannot write is a failed submit; S9 asserts the libc wording on Linux only; an empty prompt file is refused

- runner._run_cmd: the write of the prompt file was outside the cleanup
  and error handling that the exec had (merge review: claude H1, codex R1).
  A failed write now removes the partial file and returns the same 126
  failed submit an unstartable wrapper does, so the loop ends with rc 1
  and no traceback. Test S4c.
- test S9 required the literal "File too large" in stderr; bash 3.2 on
  macOS prints nothing for EFBIG, so the whole suite failed on this host
  (agy, grok, codex R2). The contract asserted everywhere is rc != 0, no
  leftover file and the portable diagnostic line; the libc wording is
  checked on Linux only.
- the wrapper accepted an empty --prompt-file and registered an empty
  prompt (claude H3); it is refused now. Test S1e.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VCgu9Fa9iT7TbJUSzdq4Ff

tests/test_issue36_prompt_transport.py

@@ -276,6 +276,44 @@ def test_s4_an_exec_error_is_a_failed_submit(runner, tmp_path, monkeypatch, caps
     assert _prompt_leftovers(tmp_path) == []
 
 
+def test_s4c_a_prompt_file_that_cannot_be_written_is_a_failed_submit(runner, tmp_path, monkeypatch, capsys):
+    """Merge review H1: the write, not only the exec, can fail (ENOSPC, EFBIG).
+    The runner must report it like an exec error and leave no partial file."""
+    lr, r = runner
+    real = lr.tempfile.NamedTemporaryFile
+
+    class _Broken:
+        def __init__(self, *a, **k):
+            self._fh = real(*a, **k)
+            self.name = self._fh.name
+        def __enter__(self):
+            self._fh.__enter__()
+            return self
+        def __exit__(self, *exc):
+            return self._fh.__exit__(*exc)
+        def write(self, _):
+            raise OSError(errno.ENOSPC, "No space left on device")
+
+    monkeypatch.setattr(lr.tempfile, "NamedTemporaryFile", _Broken)
+    ran = []
+    monkeypatch.setattr(lr.subprocess, "run", lambda *a, **k: ran.append(a) or None)
+    assert r.delegate_job("c", "claude", "Worker", "p" * 1000) is None
+    err = capsys.readouterr().err
+    assert "submit failed (exit 126)" in err and "cannot write the prompt file" in err, err
+    assert "Traceback" not in err
+    assert ran == [], "the wrapper must not be started without a prompt file"
+    assert _prompt_leftovers(tmp_path) == []
+
+
+def test_s1e_an_empty_prompt_file_is_refused(tmp_path):
+    """Merge review H3: --prompt-file with nothing in it registered an empty job."""
+    reg = tmp_path / "jobs"
+    empty = tmp_path / "empty.md"
+    empty.write_text("")
+    res = _dry_submit(tmp_path, reg, "--prompt-file", str(empty))
+    assert res.returncode != 0 and "is empty" in res.stderr, res.stdout + res.stderr
+
+
 def test_s4b_an_exec_error_on_the_busy_retry_is_reported_not_raised(runner, tmp_path, monkeypatch, capsys):
     lr, r = runner
     calls = []
@@ -599,7 +637,10 @@ def test_s9_shell_loop_removes_a_partial_prompt_file_and_says_why(tmp_path):
                        'output=$(delegate_job_safe submit --prompt "$(cat input.md)")\n')
     result = subprocess.run(["bash", str(harness)], cwd=tmp_path, capture_output=True, text=True, timeout=60)
     assert result.returncode != 0
-    assert "File too large" in result.stderr
+    if sys.platform == "linux":
+        # bash's own wording for EFBIG; macOS bash 3.2 prints nothing here
+        # (merge review N1), so only the portable line below is the contract
+        assert "File too large" in result.stderr
     leftovers = list((tmp_path / ".mam").glob("prompt-shloop-*"))
     assert not leftovers, [(p.name, p.stat().st_size) for p in leftovers]
     assert "delegate_job_safe: cannot write the prompt file" in result.stderr