fix(issue-36): create_session hands the agent a brief path, not the task; a prompt file that cannot be written is removed

Review of f26952c (codex, job cc849cbf):

- P2: create_session registered the task by file but then pasted the whole
  task into the onboarding text, which send_keys_safe hands to `herdr agent
  prompt` as one argv string. A 130,500-byte --submit-job was registered
  intact and never reached the agent (execve E2BIG). The task now goes into
  <jobs>/<id>/brief.md, as the delegate-job wrapper does, and the injected
  text names that path. The publish command and its dispatch token (#23) are
  unchanged. The other send_keys_safe callers (the wrapper's short pointer,
  stop_session's exit key) carry no task text.
- P3: delegate_job_safe's write-failure branch left the partial prompt file
  behind and logged to stdout, which callers capture. It now removes the file
  and logs to stderr.

S8 (real create_session.sh, 100 and 130,500 bytes, strace when present) and
S9 (the real function, RLIMIT_FSIZE write failure) carry the reviewer's
reproductions over; all three fail on f26952c.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.agents/skills/multi-agent-mux-loop/scripts/run_loop.sh

@@ -201,7 +201,8 @@ delegate_job_safe() {
       mkdir -p "$MAM_REAL_ROOT/.mam" \
         && prompt_file="$(mktemp "$MAM_REAL_ROOT/.mam/prompt-shloop-XXXXXX")" \
         && printf '%s' "$2" > "$prompt_file" \
-        || { log_error "delegate_job_safe: cannot write the prompt file under $MAM_REAL_ROOT/.mam"; return 1; }
+        || { [ -n "$prompt_file" ] && rm -f "$prompt_file"
+             log_error "delegate_job_safe: cannot write the prompt file under $MAM_REAL_ROOT/.mam" >&2; return 1; }
       args+=(--prompt-file "$prompt_file")
       shift 2
     else