fix: attach_command from one builder; no kill/start_command; stop closes a row-less session (Issue #71)

- attach_command is `herdr --session <server> agent attach <herdr name>`
  (`default` written too), built by lib_py.agents.sanitize.attach_command;
  create, resume and reconcile all use it. kill_command and start_command
  are no longer written, and status --json drops start_command.
- The two raw kill-session hints (create exit 43, id_order's orphan) print a
  stop-skill command with --herdr-session instead.
- stop_session.sh closes a session that has no registry row, herdr only and
  without --agent, when herdr runs it inside this workspace; another
  workspace's session is refused (exit 5), a missing one is exit 1, one that
  survives kill-session is exit 6. The registry is not changed.
- Docs: only the SKILL.md lines these changes made wrong; the skill block
  pins follow. Tests that asserted the removed fields keep their intent
  (issue50: create's own row; issue21: no control-root path in
  attach_command; tier2: the new form and the absent fields). S11 pins the
  shim to 4243da5, which changed it on purpose.

Refs #71

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

.agents/skills/multi-agent-mux-create/scripts/create_session.sh

@@ -361,7 +361,7 @@ cleanup_herdr_on_error() {
   if [ "$alive" = "1" ]; then
     _mark_row_rolled_back "running" "rollback-failed"
     echo "ERROR: rollback could not kill herdr session '$SESSION_NAME' (kill rc=$kill_rc) - it is still running." >&2
-    echo "       Clean it up with: HERDR_SESSION_NAME=${HERDR_SESSION_NAME:-default} herdr kill-session -t $SESSION_NAME" >&2
+    echo "       Clean it up with: bash .agents/skills/multi-agent-mux-stop/scripts/stop_session.sh --session $SESSION_NAME --herdr-session ${HERDR_SESSION_NAME:-default}" >&2
     exit 43
   fi
   _mark_row_rolled_back "terminated" "rolled-back"
@@ -428,11 +428,6 @@ PANE_PID=$(_herdr list-panes -t "$SESSION_NAME" -F '#{pane_pid}' 2>/dev/null ||
 PANE_CWD=$(_herdr list-panes -t "$SESSION_NAME" -F '#{pane_current_path}' 2>/dev/null || echo "$WORKSPACE")
 PANE_CMD=$(_herdr list-panes -t "$SESSION_NAME" -F '#{pane_current_command}' 2>/dev/null || echo "$AGENT")
 
-# 시작 명령
-# NOTE: this must match what `spawn()` actually ran above — env-var-driven
-# herdr server shim (HERDR_SESSION_NAME picked up by the lib.sh shim).
-START_CMD="HERDR_SESSION_NAME=${HERDR_SESSION_NAME:-default} herdr new-session -d -s \"$SESSION_NAME\" -x 140 -y 40 -c \"$WORKSPACE\" \"$CMD_FULL\""
-
 # If --onboard is specified, automatically build the onboarding prompt (if session is ready)
 if [ "$ONBOARD" = "1" ] && [ -z "$SUBMIT_JOB_PROMPT" ] && [ -z "$LAST_VISIBLE_STATUS_OVERRIDE" ]; then
   SUBMIT_JOB_PROMPT="You are a newly spawned $ROLE Team Leader agent in this workspace. Without making any non-standard pre-preparations or modifying files directly, proceed immediately to align yourself with the project context by performing the following tasks:
@@ -508,12 +503,13 @@ fi
 atomic_dump_yaml "$AGENT_SESSIONS_YAML" \
   SESSION_NAME="$SESSION_NAME" AGENT="$AGENT" NOW_ISO="$NOW_ISO" \
   HERDR_EPOCH="$HERDR_EPOCH" PANE_PID="$PANE_PID" PANE_CWD="$PANE_CWD" \
-  CMD_FULL="$CMD_FULL" START_CMD="$START_CMD" CHILD_PID="$CHILD_PID" \
+  CMD_FULL="$CMD_FULL" CHILD_PID="$CHILD_PID" \
   HERDR_SESSION_NAME="${HERDR_SESSION_NAME:-default}" \
   MAM_WS_LABEL="$MAM_WS_LABEL" \
   SESSION_UUID="$SESSION_UUID" \
   LAST_VISIBLE_STATUS_OVERRIDE="${LAST_VISIBLE_STATUS_OVERRIDE:-}" \
   DELEGATE_JOB_ID="$DELEGATE_JOB_ID" ROLE="$ROLE" CONTROL_PLANE_JSON="$CP_JSON" <<'PYEOF'
+from lib_py.agents.sanitize import attach_command
 name = os.environ['SESSION_NAME']
 agent = os.environ['AGENT']
 role = os.environ['ROLE']
@@ -567,9 +563,7 @@ entry = {
         'cmd_full': os.environ['CMD_FULL'],
         'cwd': os.environ['PANE_CWD'],
     },
-    'start_command': os.environ['START_CMD'],
-    'attach_command': f'HERDR_SESSION_NAME={server_name} herdr agent attach {name}',
-    'kill_command': f'HERDR_SESSION_NAME={server_name} herdr kill-session -t {name}',
+    'attach_command': attach_command(server_name, name),
 }
 
 if agent == 'claude':