Add follow-up reminder architecture boundaries

[nonggde]

Closes #359

Summary

• add folder-local Follow-up Reminder module boundary notes covering public API, engine, guards, fixtures, components, tests, and docs
• add data ownership notes for normalized email snapshots, advisory reminder review models, out-of-scope product data, and future integration handoff
• add an architecture boundary fixture plus Node contract test to verify folder scope, module ownership, forbidden integration areas, and state ownership

Scope

• only touches tools/v1/individual/follow-up-reminder/
• no app shell, dashboard, routing, inbox architecture, mail rendering, auth, wallet, Stellar, database, calendar, notification, provider, analytics, or design-system integration
• no production data, live network calls, server persistence, secrets, webhooks, background jobs, mailbox mutation, reminder writes, calendar writes, notification sends, or paid services
• avoids the old closed [V1][individual] Follow-up Reminder - Architecture and folder contract #359 attempt paths: README.md, specs.md, and docs/architecture.md

Validation

• node --test tools/v1/individual/follow-up-reminder/tests/architecture-boundaries.test.mjs
• npx --yes prettier --check tools/v1/individual/follow-up-reminder/docs/MODULE_BOUNDARIES.md tools/v1/individual/follow-up-reminder/docs/DATA_OWNERSHIP_CONTRACT.md tools/v1/individual/follow-up-reminder/fixtures/architecture-boundary-map.json tools/v1/individual/follow-up-reminder/tests/architecture-boundaries.test.mjs
• git diff --cached --check
• ASCII scan over changed Follow-up Reminder architecture files

Notes

• this is an architecture contract only; the existing engine, guards, fixtures, components, and Vitest suites are left unchanged