fix(session-memory): use HHMMSS slug to prevent same-minute overwrites

Custom content and no-session paths fell through to HHMM (4-char)
timestamp slug. Two /new events in the same minute would overwrite.
HHMMSS makes collisions require same-second timing.

src/hooks/bundled/session-memory/handler.ts

@@ -322,7 +322,7 @@ const saveSessionToMemory: HookHandler = async (event) => {
     // If no slug, use timestamp
     if (!slug) {
       const timeSlug = now.toISOString().split("T")[1].split(".")[0].replace(/:/g, "");
-      slug = timeSlug.slice(0, 4); // HHMM
+      slug = timeSlug.slice(0, 6); // HHMMSS — seconds prevent same-minute overwrites
       log.debug("Using fallback timestamp slug", { slug });
     }