feat(issue-50): create --onboard waits for the onboarding job to end (A)

The handshake only said the agent saw the prompt (#23). create_session.sh
now waits, after `started`, for the onboarding job to reach a terminal
status and makes that its own result, so the orchestrator learns it from
create's exit (MULTI_AGENT_RULES §6) instead of guessing from herdr's
agent_status:

- lib_py/delegation/wrapper.py: `wait-terminal` (completed|error|cancelled,
  `timeout` after --timeout, `missing` when the record is gone; stops with
  its caller via --watch-pid) and `onboard-exit` (completed 0, error or
  cancelled 44, timeout or missing 45). bash branches on the printed answer.
- lib.sh: delegate_wait_terminal / delegate_onboard_exit, thin calls.
- create_session.sh: `onboarding completed: job <id> (<n>s)` on stdout, or
  `last_visible_status: onboarding-<status>` on the row, the reason on stderr
  and `exit 44` / `exit 45` after `trap - EXIT` - the session is kept, as for
  a missing handshake. The "no started" path (error, exit 0 before) now ends
  with 44. MAM_ONBOARD_WAIT_S (default 600, floor 1) bounds the wait.
- worktree_prepare.sh: 44/45 pass through with "session and worktree kept".
- create SKILL.md exit table (44, 45) and the knob; the #42 inventory follows.
- tests/conftest.py: --onboard is the default, so ~60 sandbox create tests
  reach the new wait. The mock herdr now acts as the agent for create's
  onboarding prompt - it runs the publish commands the prompt names through
  the real publish_event.py - and MAM_ONBOARD_WAIT_S=3 caps a test that turns
  that off (MAM_MOCK_ONBOARD_HANDSHAKE=0 -> 44, `started` -> 45). The
  delegate-job wrapper's prompt has another shape and is never answered.
- tests/test_issue50_onboard.py: the decision tables, create in the sandbox
  (0 / 44 / 45, session kept, row note, trap window), the knob's integer
  rule, worktree_prepare's pass-through.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

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

@@ -15,7 +15,8 @@
 # creator session registered in the control root's .mam -> summary with BASE=.
 #
 # Exit codes: 0 ok, 1 git / submodule / create failure, 2 usage or path rule,
-# 3 refused to protect existing content (nothing changed).
+# 3 refused to protect existing content (nothing changed); 44/45 are create's
+# (onboarding error / timeout; session and worktree kept, Issue #50).
 set -euo pipefail
 
 _script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
@@ -239,7 +240,11 @@ if [ -n "$CREATOR_AGENT" ]; then
   rc=0
   env "${env_add[@]}" bash "$SKILL_DIR/multi-agent-mux-create/scripts/create_session.sh" "${args[@]}" || rc=$?
   if [ "$rc" -ne 0 ]; then
-    echo "ERROR: create_session.sh failed (rc=$rc); the worktree is kept - remove it with worktree_remove.sh --path '$WT'" >&2
+    case "$rc" in
+      44) echo "ERROR: onboarding error for '$SESSION' (create_session.sh rc=44); session and worktree kept" >&2 ;;
+      45) echo "ERROR: onboarding timeout for '$SESSION' (create_session.sh rc=45); session and worktree kept" >&2 ;;
+      *) echo "ERROR: create_session.sh failed (rc=$rc); the worktree is kept - remove it with worktree_remove.sh --path '$WT'" >&2 ;;
+    esac
     exit "$rc"
   fi
 fi