docs: document Twitch notification plugin

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

docs/USAGE.md

@@ -598,3 +598,34 @@ Polling and announcements:
 - Supports HTTP conditional requests (`ETag`, `If-Modified-Since`)
 - 5 consecutive errors doubles the poll interval (max 1 hour)
 - Subscriptions persist across bot restarts via `bot.state`
+
+### `!twitch` -- Twitch Livestream Notifications
+
+Follow Twitch streamers and get notified when they go live. Uses Twitch's
+public GQL endpoint (no API credentials required).
+
+```
+!twitch follow <username> [name]   Follow a streamer (admin)
+!twitch unfollow <name>            Unfollow a streamer (admin)
+!twitch list                       List followed streamers
+!twitch check <name>               Check status now
+```
+
+- `follow` and `unfollow` require admin privileges
+- All subcommands must be used in a channel (not PM)
+- If `name` is omitted on `follow`, it defaults to the Twitch login (lowercase)
+- Names must be lowercase alphanumeric + hyphens, 1-20 characters
+- Twitch usernames must match `[a-zA-Z0-9_]{1,25}`
+- Maximum 20 streamers per IRC channel
+
+Polling and announcements:
+
+- Streamers are polled every 2 minutes by default
+- On `follow`, the current stream state is recorded without announcing
+- Announcements fire on state transitions: offline to live, or new stream ID
+- Format: `[name] is live: Stream Title (Game) -- https://twitch.tv/login`
+- Game is omitted if not set; titles are truncated to 80 characters
+- 5 consecutive errors doubles the poll interval (max 1 hour)
+- Subscriptions persist across bot restarts via `bot.state`
+- `list` shows live/error status indicators next to each streamer
+- `check` forces an immediate poll and reports current status