Add deadline detector architecture contract

[nonggde]

Closes #479

Summary

• add a folder-local architecture contract for Deadline Detector
• document data ownership, fixture/runtime boundaries, dependency rules, and future reminder/calendar adapter constraints
• add a zero-dependency Node architecture contract test for required docs, template cleanup, import isolation, and forbidden app/provider imports

Scope

• only touches tools/v2/individual/deadline-detector/
• no app shell, dashboard, routing, inbox, auth, wallet, Stellar, database, reminder, calendar, notification, provider, or design-system integration
• no production data, live network calls, persistence, secrets, or side effects

Validation

• node --test tools/v2/individual/deadline-detector/tests/architecture-contract.test.mjs
• node --test tools/v2/individual/deadline-detector/tests/deadline-fixtures.test.mjs
• npx --yes prettier --check on changed Deadline Detector files
• git diff --check
• ASCII scan over tools/v2/individual/deadline-detector