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/scripts/install_phase.sh

@@ -81,7 +81,14 @@ MON="${3:-0}"
 TMPD="${4:-}"
 trap '[ -z "$TMPD" ] || rm -rf "$TMPD"' EXIT
 
-set_state installing || { echo "ERROR: cannot write $BK/state" >&2; release_marker; exit 1; }
+if ! set_state installing; then
+  # Nothing was installed yet: drop the backup and put the monitor back as phase 1 found it.
+  echo "ERROR: cannot write $BK/state; nothing was changed" >&2
+  rm -rf "$BK"
+  restart_old_monitor "$MON"
+  release_marker
+  exit 1
+fi
 URL="${MAM_INSTALLER_URL:-https://git.godopu.com/tmpl/multi-agent-mux/raw/branch/main/deploy/install.sh}"
 echo "install: $URL"
 irc=0