docs(issue-23): the create skill documents the preserve-on-unacknowledged-onboarding rule and MAM_ONBOARD_ACK_S

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.agents/skills/multi-agent-mux-create/SKILL.md

@@ -104,7 +104,13 @@ The bootstrapped server receives a **sanitized copy of the caller's environment*
    the agent's behalf, so a job that shows `started` really was picked up. A
    send_keys rc 4 ("Enter not seen accepted", where a still-starting TUI lands) is
    a warning; rc 1/2/3 still abort at once. No `started` within the ack timeout
-   fails the job and exits 1 — the prompt is never re-injected.
+   marks the job `error` and warns, but **keeps the session**: the pane is up and
+   the row is valid, only the handshake never landed, and killing a live agent
+   over a slow onboarding would destroy more than it protects (the same rule
+   this script applies to a TUI that never signalled ready). Re-send with
+   `multi-agent-mux-delegate-job submit` once the agent is up; the prompt is
+   never re-injected automatically. `MAM_ONBOARD_ACK_S` overrides how long that
+   wait is — distinct from the wrapper's `MAM_INJECT_ACK_GRACE_S`.
 
    **Start-up dialogs** are answered for every agent, not only claude (#24). The
    blocking-modal signatures and their accept keys live in each agent adapter