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

.agents/skills/lib_py/loop/runner.py

@@ -469,12 +469,26 @@ class LoopRunner:
             # MAX_ARG_STRLEN (131,072 bytes on Linux), and a review prompt
             # carries the whole diff. The wrapper blocks until the job ends,
             # so nothing reads the file after it returns.
-            self.mam_dir.mkdir(parents=True, exist_ok=True)
-            with tempfile.NamedTemporaryFile("w", encoding="utf-8", dir=self.mam_dir,
-                                             prefix=f"prompt-{self.loop_id}-", suffix=".md",
-                                             delete=False) as fh:
-                fh.write(p_text)
-                prompt_file = fh.name
+            prompt_file = ""
+            try:
+                self.mam_dir.mkdir(parents=True, exist_ok=True)
+                with tempfile.NamedTemporaryFile("w", encoding="utf-8", dir=self.mam_dir,
+                                                 prefix=f"prompt-{self.loop_id}-", suffix=".md",
+                                                 delete=False) as fh:
+                    prompt_file = fh.name
+                    fh.write(p_text)
+            except OSError as e:
+                # The same class the shell paths handle (merge review H1): a
+                # partial file is removed and the submit fails like an exec
+                # error would, instead of the exception reaching the loop.
+                if prompt_file:
+                    try:
+                        os.unlink(prompt_file)
+                    except OSError:
+                        pass
+                msg = (f"cannot write the prompt file under {self.mam_dir}: {e} "
+                       f"(prompt {len(p_text.encode('utf-8'))} bytes)")
+                return subprocess.CompletedProcess(["bash", str(wrapper_script), "submit"], 126, "", msg)
             cmd = [
                 "bash", str(wrapper_script), "submit",
                 "--agent-session", self.session_label(agent_session),