feat(core): typed REST client + error model + idempotency

[gabvega08]

Summary

• Add typed REST client (PactoApiClient) for listings, quotes, and escrows, exposed via client.api(session) using the session clientSecret.
• Implement error hierarchy: PactoAuthError, PactoRateLimitError, PactoEscrowError, PactoApiError with HTTP-to-class mapping via errorFromResponse.
• Add HTTP layer with Idempotency-Key on write operations (reused across retries), exponential backoff + jitter for 5xx/429/network failures.

Closes #13

Test plan

• npm run lint
• npm run type-check
• npm run test
• Idempotency-Key reused on retry (same key on 500 → 200)
• Error mapping: 401/403 → Auth, 429 → RateLimit, escrow → EscrowError
• GET requests do not send Idempotency-Key
• Manual: create session, call client.api(session).escrows.create(...) against Gateway once proxy routes exist