Fix brainstorm server crashing on shutdown when the session dir is gone

[zigggy-stardust]

Problem

shutdown() in skills/brainstorming/scripts/server.cjs writes state/server-stopped without checking that STATE_DIR still exists:

function shutdown(reason) {
  console.log(JSON.stringify({ type: 'server-stopped', reason }));
  const infoFile = path.join(STATE_DIR, 'server-info');
  if (fs.existsSync(infoFile)) fs.unlinkSync(infoFile);
  fs.writeFileSync(                                   // ← throws ENOENT
    path.join(STATE_DIR, 'server-stopped'),
    JSON.stringify({ reason, timestamp: Date.now() }) + '\n'
  );
  watcher.close();                                    // never runs
  clearInterval(lifecycleCheck);                      // never runs
  for (const socket of clients) { ... }               // never runs
  server.close(() => process.exit(0));                // never runs
}

If the session directory is removed while the server sits idle, the write throws from inside the lifecycleCheck interval. The throw is not caught, so every cleanup step below it is skipped and the process dies on an uncaught exception with exit code 1 instead of exiting 0.

This is not just a cosmetic exit code — the watcher, the interval, the WebSocket sockets and server.close() are all bypassed.

How I hit it

A brainstorming session finished, the design was committed, and I deleted the scratch .superpowers/ directory. The server had already begun a normal idle timeout shutdown, and supervision reported it as a failed process:

{"type":"server-stopped","reason":"idle timeout"}
Error: ENOENT: no such file or directory, open '.../state/server-stopped'
    at shutdown (.../skills/brainstorming/scripts/server.cjs:620:8)
    at Timeout._onTimeout (.../skills/brainstorming/scripts/server.cjs:642:59)

Minimal reproduction

D=$(mktemp -d); mkdir -p "$D/content" "$D/state"
BRAINSTORM_DIR="$D" BRAINSTORM_PORT=0 \
BRAINSTORM_IDLE_TIMEOUT_MS=1500 BRAINSTORM_LIFECYCLE_CHECK_MS=300 \
  node skills/brainstorming/scripts/server.cjs & P=$!
sleep 1.2; rm -rf "$D/state"; wait $P; echo "exit=$?"
# before: exit=1 + ENOENT
# after:  exit=0

Fix

State-dir bookkeeping becomes best effort, matching the pattern already used a few lines below for PORT_FILE (try { fs.writeFileSync(PORT_FILE, ...) } catch (e) { /* best effort */ }):

• skip the bookkeeping when STATE_DIR is gone, wrap it in try/catch, and always fall through to the real cleanup
• guard watcher.close() too, since CONTENT_DIR disappears in the same scenario
• make shutdown() idempotent so a second lifecycle tick cannot call server.close() twice

The sentinel is deliberately not recreated when the directory is missing — nothing is left to consume it, and re-creating a tree the user just deleted would be surprising.

Tests

Adds one regression test to tests/brainstorm-server/lifecycle.test.js (port 3420, following the existing style in that file): remove the session dir while the server is idle, then assert a clean exit 0 with no ENOENT on stderr.

Verified both directions:

	new test	full npm test
without the fix	FAIL (13 passed, 1 failed)	fails
with the fix	PASS	see below

Behaviour on the normal path is unchanged — with the state dir present, the sentinel is still written and the process still exits 0.

tests/brainstorm-server $ npm test
--- Results: 32 passed, 0 failed ---     ws-protocol
--- Results: 15 passed, 0 failed ---     helper
--- Results:  3 passed, 0 failed ---     browser-launcher
--- Results: 20 passed, 0 failed ---     auth
--- Results:  7 passed, 0 failed ---     branding
--- Results: 33 passed, 0 failed ---     server
--- Results: 14 passed, 0 failed ---     lifecycle   ← +1 new test
--- Results:  4 passed, 0 failed ---     start-server.sh
--- Results:  6 passed, 1 failed ---     stop-server.sh

Note on the one remaining failure

stop-server.sh :: persistent stop writes server-stopped fails, but it is pre-existing and unrelated to this change. I reproduced it on pristine main (b36e082) in the same environment, before and after applying the patch — identical 6 passed, 1 failed both times. I left it alone rather than widen the scope of this PR.

Environment: macOS 25.6.0 (arm64), Node v26.5.1.