docs(phase5c): enrollment communications documentation + Google Workspace runbook

- PHASE5C_ENROLLMENT_COMMUNICATIONS.md: the full design doc.
- RUNBOOK_GOOGLE_WORKSPACE_EMAIL.md: every REQUIRES GOOGLE WORKSPACE ADMIN ACTION step
  (mailbox, service account, domain-wide delegation scoped to gmail.send only, SPF/DKIM/DMARC),
  environment variables, testing procedure, production secret handling, rotation, revocation,
  troubleshooting, quotas.
- RUNBOOK_STUDENT_ACTIVATION.md: day-to-day operation of the pipeline from /admin.
- WEBSITE_ENROLLMENT_INTEGRATION.md: explicit website-vs-RootNode responsibility boundary,
  the new contact_email field, and closes the 'RootNode doesn't send activation email yet'
  known limitation this doc previously documented.
- RUNBOOK_SELF_PACED_ENROLLMENT.md, RUNBOOK_STUDENT_ACCOUNT_LIFECYCLE.md: point at the new
  automatic-email behavior and the Resend activation control.
- REMOTE_AND_SELF_PACED_ARCHITECTURE.md, ENGINEERING_SOP.md: close the enrollment-
  communication gap; new ENGINEERING_SOP §16.
- PRODUCTION_VALIDATION_CHECKLIST.md, README.md, platform/README.md: cross-references and a
  Phase 5C production-validation section.

docs/RUNBOOK_SELF_PACED_ENROLLMENT.md

@@ -26,6 +26,10 @@ Optional start/end dates enforce access windows server-side — a future start d
 access until then; an end date cuts off new Lab launches after it while leaving prior progress
 visible.
 
+As of Phase 5C, this also automatically emails the student — an activation link if they've never
+had credentials, or an enrollment confirmation if they already do. See
+`docs/RUNBOOK_STUDENT_ACTIVATION.md` for checking delivery status and resending.
+
 **Activate a pending enrollment** (Programs → \[program\] → Enrollments table → Activate):
 
 Only needed if you enrolled as `pending`. This is the moment a student actually gains entry.