Fix query string separator and missing closing paren
- atproto/xrpc.rkt: use "&" not "," to join query params. The
comma produced invalid URLs like ?limit=10,actor=foo instead
of ?limit=10&actor=foo. Unnoticed so far as all current calls
pass only one param.
- bluesky/open-app.rkt: add missing ")" in feed option printf
format string, closing the option's parenthesized command span.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>