feat(gateway): OpenAI-compatible custodial gateway

New multi-tenant gateway package: external developers point any OpenAI SDK at
the marketplace. Each API key is its own custodial buyer wallet (AES-256-GCM
sealed under GATEWAY_MASTER_KEY); every request posts an on-chain escrow from
that user's wallet.

- POST /openai/v1/chat/completions (one escrow/call, llm.text.generate.v1,
  buffered + pseudo-stream), chat sessions (one escrow/session, llm.chat.v1,
  real SSE), GET /openai/v1/models, /signup, /account, /account/withdraw.
- Settles by resolving the Submitted ref via the indexer + awaited Accept;
  state-aware reclaim sweeper + per-wallet auto-consolidation for collateral.
- Reuses the buyer Marketplace SDK via new buyer exports ./sdk; new shared
  buildWithdrawTx (custodial exit). Runs via tsx; Dockerfile + mainnet compose.

Design in docs/gateway.md. Typecheck passes for all packages; 20 vitest tests
pass. Not yet exercised against a live chain.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pnpm-workspace.yaml

@@ -1,6 +1,7 @@
 packages:
   - "packages/*"
   - "buyer"
+  - "gateway"
   - "supplier"
   - "indexer"
   - "indexer-ui"