Add deadline detector security guard boundaries

[nonggde]

Closes #482

Summary

• add folder-local Deadline Detector guard helpers for malformed messages, sender/header injection, unsafe text, attachments, histories, and oversized batches
• add hostile input fixtures covering path-like ids, unsupported source types, CRLF sender injection, invalid timestamps, unsafe timezones, and malformed personal-data flags
• document security assumptions, unsafe input handling, performance limits, and future adapter requirements

Scope

• only touches tools/v2/individual/deadline-detector/
• no app shell, dashboard, routing, inbox, compose, auth, wallet, Stellar, database, reminder, calendar, notification, provider, AI provider, or design-system integration
• no production data, live network calls, server persistence, secrets, webhooks, background jobs, send actions, save actions, reminder writes, calendar writes, or mailbox mutation

Validation

• node --test tools/v2/individual/deadline-detector/tests/deadline-guards.test.mjs
• node --test tools/v2/individual/deadline-detector/tests/deadline-fixtures.test.mjs
• npx --yes prettier --check tools/v2/individual/deadline-detector/guards/deadline-guards.mjs tools/v2/individual/deadline-detector/fixtures/hostile-deadline-inputs.json tools/v2/individual/deadline-detector/docs/SECURITY_AND_PERFORMANCE.md tools/v2/individual/deadline-detector/tests/deadline-guards.test.mjs
• git diff --cached --check
• ASCII scan over changed Deadline Detector files

Push note

• normal HTTPS git push to the fork hung with no output and was stopped after confirming the matching git processes
• the fork branch was created from upstream main and updated through the GitHub Contents API with the same changed file contents