Fix owner-PID false positive when owner runs as different user

ownerAlive() treated EPERM (permission denied) the same as ESRCH
(process not found), causing the server to self-terminate within 60s
whenever the owner process ran as a different user. This affected WSL
(owner is a Windows process), Tailscale SSH, and any cross-user
scenario.

The fix: `return e.code === 'EPERM'` — if we get permission denied,
the process is alive; we just can't signal it.

Tested on Linux via Tailscale SSH with a root-owned grandparent PID:
- Server survives past the 60s lifecycle check (EPERM = alive)
- Server still shuts down when owner genuinely dies (ESRCH = dead)

Fixes #879

skills/brainstorming/scripts/server.cjs

@@ -313,7 +313,7 @@ function startServer() {
 
   function ownerAlive() {
     if (!OWNER_PID) return true;
-    try { process.kill(OWNER_PID, 0); return true; } catch (e) { return false; }
+    try { process.kill(OWNER_PID, 0); return true; } catch (e) { return e.code === 'EPERM'; }
   }
 
   // Check every 60s: exit if owner process died or idle for 30 minutes