feat: live class sessions, CE workflow, and docs (#683)

Adds live-class session scheduling on lessons (join link, date/time,
timezone) with automatic 24h and 1h reminder emails plus a calendar
invite (.ics), a time-change notification with an updated invite, a
teacher-authored welcome email and session invite on enrollment, and a
"Notify students" quiz nudge.

LMS gets an upcoming live-sessions card and Join buttons, plus a
reusable live-session card component (3 states) in @cio/ui with a
Storybook story. The sidebar content tree now reflects sequential
progression locking like the content list, and the course-completion
confetti regression is fixed.

Org-based emails no longer reference ClassroomIO by name. Docs add
live-class, welcome-email, and course-progression guides, expand the
certificates guide (design, signatures, corrected labels), move
plan-required callouts to the top, flatten how-to-guides to the docs
root with redirects, and add screenshots. Includes an add-docs-image
skill.

packages/email/src/index.ts

@@ -32,6 +32,9 @@ export type { EmailId, EmailSchemaFor } from './utils/types';
 export { deliverEmail } from './send';
 export { buildEmailFromName } from './utils/functions/email-helpers';
 export { getDefaultTemplate } from './templates/default';
+export { buildEmailBranding, resolveThemeColor, ZEmailBranding } from './core/branding';
+export { buildSessionIcs, type SessionIcsInput } from './ics';
+export type { EmailBranding } from './core/branding';
 
 // Export email registry for introspection (optional)
 export { EmailRegistry } from './core/registry';