…y protection

Implements the @openthreads/trust package and server integration for Issue 14.

packages/trust:
- JWS signing via Web Crypto API (ES256/ECDSA P-256) — no external deps
  signIntent/signResponse bind intent → response into a verifiable evidence chain
- Replay protection: ReplayGuard with nonce store + timestamp validation
- Audit logging: AuditLogger + InMemoryAuditStorage + AuditStorageAdapter interface
- Strong auth: TOTP (RFC 6238 via Web Crypto HMAC-SHA1), WebAuthn challenge gen/verify
- AuthChallengeManager: issue + verify challenges (webauthn, totp, sms_otp)
- TrustLayerManager: single entry point wiring all subsystems together
  Auto-generates ES256 key pair; enabled=false means zero overhead

packages/server:
- GET /api/audit: query audit log (turnId, threadId, eventType, date range filters)
- POST /api/form/:key/auth: issue auth challenge before form submission
- PUT  /api/form/:key/auth: verify challenge (TOTP code or WebAuthn assertion)
- Form GET: returns requiresAuth=true + emits intent_rendered audit event
- Form POST: requires verified challengeId when TRUST_LAYER_ENABLED=true
- TrustLayerManager singleton (globalThis) with MongoDB-backed audit storage
- audit_log MongoDB collection with indexes

Co-authored-by: claude[bot] <claude[bot]@users.noreply.github.com>