feat: add multi-gateway smart routing

[andrei-hasna]

Summary

• add generic OpenAI-compatible gateway support with baseUrlEnv, custom auth, and static/env-derived headers
• add first-class presets/examples for OpenRouter Auto, Vercel AI Gateway, LiteLLM Proxy, Portkey, Cloudflare AI Gateway, Helicone AI Gateway, and Kong AI Gateway
• add policy-first smart routing with cost/quality/latency/throughput scoring, deterministic fallback, capability/context filters, and explainable route scores
• map OpenRouter and Vercel provider options narrowly while stripping gateway-only fields for direct providers
• document why deterministic smart routing lives in open-gateway while open-router remains a future companion package

Verification

• bun run typecheck
• bun test
• bun test tests/smoke.test.ts
• bun run build
• bun run src/cli/index.ts validate --config gateway.config.example.json
• bun run src/cli/index.ts validate --config gateway.config.no-china.example.json
• bun run src/cli/index.ts validate --config gateway.config.china.example.json

Notes

• Reconciled adversarial review findings: unpriced models now fail price ceilings closed; OpenRouter ZDR/logging policy cannot be weakened by client provider options; required header envs are route eligibility checks.
• npm publish was not attempted because the documented release gate requires live provider smoke, and this task requested mock-provider smoke verification.