fix(self-update): stop a stalled status from blocking every later update

A Homebrew node upgrade under a long-running server deletes the versioned
Cellar path the server passes as --node, so every status write from the
updater failed. The update itself still built and restarted (npm and the
build use node from PATH), but update-status.json stayed "queued" forever.
The boot reconcile ran one minute after the restart, inside its 15 min
window, and isInFlight() had no age limit, so "An update is already in
progress." blocked every later update until the next server restart.

- self-update.sh falls back to node on PATH when --node is not executable.
- expireStalledStatus() (pure) fails an in-flight status whose last write
  is older than the stale window; applied on every read (start + status
  poll) and persisted. The live updater heartbeats every few seconds, so a
  running update never trips it.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

scripts/self-update.sh

@@ -74,6 +74,15 @@ echo "[self-update] $(date) start tag=$TAG supervisor=$SUPERVISOR repo=$REPO"
 export PATH="$(dirname "$NODE"):$HOME/.local/bin:$HOME/.npm-global/bin:/usr/local/bin:/opt/homebrew/bin:$PATH"
 export GIT_TERMINAL_PROMPT=0
 
+# --node is the server's process.execPath, a VERSIONED path (Homebrew resolves it
+# into Cellar/node/<ver>/). A `brew upgrade node` under a long-running server
+# deletes it, and every status write then failed, so the status stayed "queued"
+# forever. Fall back to whatever node is on PATH.
+if [ ! -x "$NODE" ]; then
+  echo "[self-update] WARN: $NODE is not executable, falling back to node on PATH"
+  NODE="$(command -v node || echo node)"
+fi
+
 TO_VERSION="${TAG##*@}"   # codeman@0.9.4 → 0.9.4 (tag is validated upstream)
 STASH_REF=""
 MANUAL_CMD=""