fix(issue-36): the legacy shell loop and lib.sh's submit helpers pass prompts by file

The two remaining exec points that carried a prompt as one argv string
(capped at 131,072 bytes):

- run_loop.sh (MAM_LOOP_PY=0): delegate_job_safe, which all eleven shell
  delegations go through, turns `--prompt <text>` into `--prompt-file` under
  the control root's .mam/ and removes the file once the wrapper returns. Its
  failure lines go to stderr: callers capture stdout, and under set -e a
  failed submit ends the loop at that line with the wrapper's code, so the
  cause was being swallowed.
- lib.sh delegate_submit_job and its legacy twin (create_session's onboarding
  and --submit-job): the prompt is written under .mam/ and handed on as
  --prompt-file; lib_py.delegation.cli submit accepts --prompt-file and
  forwards it to the registry. --prompt keeps working everywhere.

S6/S6b push a 140,000-byte prompt through the CLI and both helpers; S7 runs
the real run_loop.sh with a 140,000-byte plan and a 150,000-byte diff; S7b
fixes a nonzero exit when the implementation or compliance job cannot be
registered. All six fail on 14d11bf.

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

.agents/skills/lib.sh

@@ -2032,22 +2032,36 @@ _delegate_script() {
   printf '%s\n' "$(find "$skill_dir" -name "$name" 2>/dev/null | head -n 1 || true)"
 }
 
+# _delegate_prompt_file <prompt> — write the prompt to a new file under the
+# control root's .mam/ and echo its path. Issue #36: a prompt handed to an exec
+# as one argv string is capped at 131,072 bytes; the caller removes the file.
+_delegate_prompt_file() {
+  local dir="${WORKSPACE_ROOT:-$PWD}/.mam" f
+  mkdir -p "$dir" || return 1
+  f="$(mktemp "$dir/prompt-submit-XXXXXX")" || return 1
+  printf '%s' "$1" > "$f" || { rm -f "$f"; return 1; }
+  printf '%s\n' "$f"
+}
+
 # delegate_submit_job <prompt> <agent> <agent_session>
 #
 # Register a job in the multi-agent-mux-delegate-job registry. Prints the new JID on stdout.
 _delegate_submit_job_legacy() {
   local prompt="$1" agent="$2" session="$3"
-  local py_bin registry_py
+  local py_bin registry_py prompt_file rc=0
   py_bin="$(_delegate_py_bin)"
   registry_py="$(_delegate_script registry.py)"
   if [ -z "$registry_py" ] || [ ! -f "$registry_py" ]; then
     echo "ERROR: multi-agent-mux-delegate-job registry.py not found under .agents/skills/" >&2
     return 1
   fi
+  prompt_file="$(_delegate_prompt_file "$prompt")" || { echo "ERROR: cannot write the prompt file" >&2; return 1; }
   "$py_bin" "$registry_py" register \
-    --prompt "$prompt" \
+    --prompt-file "$prompt_file" \
     --agent "$agent" \
-    --agent-session "$session"
+    --agent-session "$session" || rc=$?
+  rm -f "$prompt_file"
+  return $rc
 }
 
 # delegate_publish_event <job_id> <event> [detail]
@@ -2152,7 +2166,11 @@ delegate_submit_job() {
     echo "ERROR: multi-agent-mux-delegate-job registry.py not found under .agents/skills/" >&2
     return 1
   fi
-  _delegation_py submit --prompt "$prompt" --agent "$agent" --agent-session "$session"
+  local prompt_file rc=0
+  prompt_file="$(_delegate_prompt_file "$prompt")" || { echo "ERROR: cannot write the prompt file" >&2; return 1; }
+  _delegation_py submit --prompt-file "$prompt_file" --agent "$agent" --agent-session "$session" || rc=$?
+  rm -f "$prompt_file"
+  return $rc
 }
 
 # delegate_publish_event <job_id> <event> [detail]