s/b: poll for session directory after creation

FUSE entry cache may not reflect the new session directory immediately
after the write to s/new returns. Poll until it appears, mirroring the
approach in s/sh.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/s/b

@@ -89,6 +89,11 @@ backend=${backend}"
 model=${model}"
     printf '%s\n' "$kv" > "$MOUNT/s/new" || die "failed to create session"
 
+    # Poll until the session directory is visible (FUSE entry cache may lag).
+    local deadline=$(( $(date +%s) + 5 ))
+    until [ -d "$B" ] || [ "$(date +%s)" -ge "$deadline" ]; do sleep 0.05; done
+    [ -d "$B" ] || die "timeout waiting for session $id"
+
     printf '%s\n' "$prompt" > "$B/prompt" || die "failed to submit prompt"
 
     if [ "$bg" -eq 1 ]; then