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-create/scripts/create_session.sh

@@ -622,6 +622,19 @@ if [ -n "$DELEGATE_JOB_ID" ]; then
   # dispatch (trial-2 finding: without them the token-checking ack below could
   # never be satisfied by a real agent, and every onboarding sat out 180 s).
   pub="$(delegate_publish_cmd "$DELEGATE_JOB_ID" "$DELEGATE_JOB_TOKEN" "$DELEGATE_JOB_ATTEMPT")"
+  # Issue #36: the task goes into a brief in the job's directory and the pane
+  # gets only its path, as the delegate-job wrapper does. The injected text is
+  # one argv string to herdr (capped at 131,072 bytes), so the whole task
+  # plus this preamble could not reach the agent.
+  task_reg_dir="${MAM_JOBS_DIR:-${WORKSPACE_ROOT:-$PWD}/.mam/jobs}"
+  [[ "$task_reg_dir" == /* ]] || task_reg_dir="$PWD/$task_reg_dir"
+  task_brief="$task_reg_dir/$DELEGATE_JOB_ID/brief.md"
+  if ! { mkdir -p "${task_brief%/*}" && printf '%s' "$SUBMIT_JOB_PROMPT" > "$task_brief"; }; then
+    echo "ERROR: cannot write the task brief $task_brief" >&2
+    delegate_publish_event "$DELEGATE_JOB_ID" error "task brief could not be written" "$DELEGATE_JOB_TOKEN" "$DELEGATE_JOB_ATTEMPT"
+    JOB_TERMINALIZED=1
+    exit 1
+  fi
   instructions="Your job_id is \"$DELEGATE_JOB_ID\" (the one just registered for THIS delegation).
 
 On start run:        $pub --event started.
@@ -629,7 +642,7 @@ On progress (optional): $pub --event progress --detail '<short status>'.
 On success run:      $pub --event completed --detail '<one-line summary>'.
 On failure run:      $pub --event error     --detail '<one-line reason>'.
 
-Task: $SUBMIT_JOB_PROMPT"
+Task: read the complete task in $task_brief and follow it."
 
   # Inject instructions into the herdr pane
   rc=0