Add legal review security performance guards

[nonggde]

Closes #617

Summary

• add folder-local security and performance guard helpers for Legal and Compliance Review Flag inputs
• add synthetic hostile/large review fixtures covering malformed input, secret-like redaction, truncation, attachment limits, and history limits
• document threat assumptions, unsafe input handling, performance budgets, and out-of-scope integrations
• add a zero-dependency Node test suite for the guard behavior and documentation constraints

Scope

• only touches tools/v2/team/legal-and-compliance-review-flag/
• no app shell, routing, auth, wallet, Stellar, database, inbox, mail rendering, notification, assignment write, external legal/compliance provider, ticketing, storage provider, or design-system integration
• no production data, live network calls, persistence, secrets, or side effects

Validation

• node --test tools/v2/team/legal-and-compliance-review-flag/tests/security-performance-guards.test.mjs
• npx --yes prettier --check on the changed Legal and Compliance Review Flag files
• git diff --check
• ASCII scan over tools/v2/team/legal-and-compliance-review-flag