feat: add bulk notification operations

[eslerm]

Add bulk notification mutations: archive-all, mark-read-all, mark-unread-all, snooze-all, unsnooze-all.

Scope

• 6 new mutations extending existing notification commands
• No new queries or filter infrastructure needed
• Estimated ~520 LOC

Open Questions

• Should bulk operations require --yes confirmation flag?
• What filtering do the bulk mutations support? (by type? team? date?)
• Is notificationCategoryChannelSubscriptionUpdate useful for CLI?

See planning/06-bulk-notifications.md