fix(update): a state write failure before the install restarts the monitor; the exit table names the conversion code (Issue #72, review notes)

install_phase.sh exited when it could not mark the backup `installing`,
before the installer ran, without starting the monitor phase 1 had stopped
(decision 2) and leaving the backup behind. Nothing was changed at that
point, so it now removes the backup, restarts the monitor if it ran,
releases the marker and exits 1.

SKILL.md's exit row said 1 for every failure; a failed registry conversion
after a good install passes the converter's nonzero code through, and a
cancelled prompt exits 0.

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

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

@@ -27,4 +27,4 @@ bash .agents/skills/multi-agent-mux-update/scripts/update.sh --yes
 | Monitor | stopped first, started again afterwards only if it ran (`MAM_NO_SUBSCRIBER_DAEMON=1`: not started). |
 | Install | the installer at `MAM_INSTALLER_URL` (default: Gitea `main`) runs in place; `.mam` is kept. On failure `.agents/`, `.mam_deploy/`, root files, `.venv`, the installer's `.mam` files and the registry are restored from `.mam/update-backup/`. |
 | Registry | converted by the new code (`lib_py.registry_format`); a failed conversion keeps the install and exits nonzero. |
-| Exit | `0` updated, `1` refused or failed, `2` bad arguments. |
+| Exit | `0` updated (or cancelled at the prompt), `1` refused or failed, `2` bad arguments; a failed registry conversion after a good install exits with the converter's nonzero code. |