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/multi-agent-mux-delegate-job/multi-agent-mux-delegate-job

@@ -190,6 +190,7 @@ cmd_submit() {
     [[ -z "$PROMPT" ]] || { echo "submit takes --prompt or --prompt-file, not both" >&2; exit 1; }
     [[ "$PROMPT_FILE" == /* ]] || PROMPT_FILE="$(pwd)/$PROMPT_FILE"
     [[ -f "$PROMPT_FILE" && -r "$PROMPT_FILE" ]] || { echo "submit: cannot read --prompt-file $PROMPT_FILE" >&2; exit 1; }
+    [[ -s "$PROMPT_FILE" ]] || { echo "submit: --prompt-file $PROMPT_FILE is empty" >&2; exit 1; }
     PROMPT="$(<"$PROMPT_FILE")"
   fi
   [[ -n "$PROMPT" || -n "$PROMPT_FILE" ]] || { echo "submit requires --prompt or --prompt-file" >&2; exit 1; }