Adds two improvements from pi Zulip extension lessons:
1. _ensure_stream_subscriptions() — checks at connect time if the bot
is subscribed to the primary stream and general-chat; auto-subscribes
if missing. Without this, bots with 0 subscriptions can't receive
stream events (DMs only).
2. selftest check #9 — verifies stream subscriptions and reports
count + names. Catches the 'zero subscriptions' failure mode that
was a major debugging bottleneck in the pi extension.
Abiba (pi)
15f94e3bcb