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.

README.md

@@ -171,6 +171,11 @@ its core:
   boundary — server-to-server only, no browser session involved — is what the future public
   `rootnodeacademy.com` website/checkout will call to create these enrollments; see
   `docs/PHASE5B_SELF_PACED_ENROLLMENT.md` and `docs/WEBSITE_ENROLLMENT_INTEGRATION.md`.
+- As of Phase 5C, RootNode actually tells students what's happening — real activation,
+  enrollment-confirmation, and password-reset email, delivered asynchronously through a
+  provider-neutral notification worker (Google Workspace via the Gmail API in production, a
+  deterministic dev provider everywhere else — no test/dev run ever emails a real person). See
+  `docs/PHASE5C_ENROLLMENT_COMMUNICATIONS.md` and `docs/RUNBOOK_GOOGLE_WORKSPACE_EMAIL.md`.
 
 **Never publish the raw privileged container** — front it with isolation (that's what the
 Gateway is). The one-container `docker run --privileged -p 7554:80 ...` command above stays
@@ -208,7 +213,8 @@ Key properties of the platform (not this image):
 Production topology and what's built vs. not yet: `docs/LAB_ARCHITECTURE.md`,
 `docs/PHASE1_PLATFORM_FOUNDATION.md`, `docs/PHASE2A_GATEWAY_AND_STUDENT_FLOW.md`,
 `docs/PHASE3A_IDENTITY_AND_INSTRUCTOR_ACCESS.md`, `docs/PHASE3B_LEARNING_AND_ASSESSMENT.md`,
-`docs/PHASE5A_STUDENT_IDENTITY_REMOTE_CLASSROOM.md`, `docs/PHASE5B_SELF_PACED_ENROLLMENT.md`.
+`docs/PHASE5A_STUDENT_IDENTITY_REMOTE_CLASSROOM.md`, `docs/PHASE5B_SELF_PACED_ENROLLMENT.md`,
+`docs/PHASE5C_ENROLLMENT_COMMUNICATIONS.md`.
 
 The production infrastructure package (Proxmox/Ansible/Terraform/firewall/monitoring/backup/CI)
 that runs all of this on real hardware lives in `infra/` — see