feat: migrate agent to mpcp-gateway-client (phase 4b)

.env.example

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+# ── Agent identity (display / grant metadata) ───────────────────────────────
+AGENT_ACTOR_ID=0x...
+
+# ── Trust Gateway (mpcp-gateway) — required ───────────────────────────────────
+MPCP_GATEWAY_URL=http://localhost:8080
+MPCP_GATEWAY_API_KEY=replace-with-owner-api-key
+
+# Purpose on every GatewaySession.fetch — must match issued grant purposes
+# MPCP_SESSION_PURPOSE=inference:llm
+
+# ── MPCP Policy Authority ────────────────────────────────────────────────────
+POLICY_AUTHORITY_URL=http://localhost:3100
+
+# ── Grant persistence ────────────────────────────────────────────────────────
+GRANT_FILE=./grant.json
+
+# ── Inference ────────────────────────────────────────────────────────────────
+HYPERBOLIC_URL=https://hyperbolic-x402.vercel.app/v1/chat/completions
+HYPERBOLIC_MODEL=meta-llama/Meta-Llama-3.1-405B-Instruct
+
+# ─────────────────────────────────────────────────────────────────────────────
+# Web app (web/.env) — signer UI for demos; agent server uses the vars above.
+# ─────────────────────────────────────────────────────────────────────────────

.gitignore

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+node_modules/
+dist/
+.env
+grant.json
+web/.next/
+web/tsconfig.tsbuildinfo

README.md

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+# mpcp-hyperbolic-agent
+
+An AI agent that uses [MPCP](https://mpcp-protocol.github.io/spec/) to enforce human-approved spending limits on [Hyperbolic](https://hyperbolic.xyz/) inference calls.
+
+The agent pays for its own LLM inference via [x402](https://github.com/coinbase/x402). MPCP provides the spending guardrails: a human issues a PolicyGrant defining a budget ceiling and allowed purposes. The agent cannot overspend or operate outside those bounds — cryptographically, not just by convention.
+
+---
+
+## Phase 1 — Agent-side enforcement
+
+This repo implements Phase 1: the minimal loop that proves MPCP correctly bounds a real agent making real paid inference calls.
+
+```
+Human / Operator
+    ↓  POST /grants  (one-time)
+mpcp-policy-authority
+    ↓  SignedPolicyGrant  (ceiling: $1 USDC, purposes: ["compute:inference"])
+Agent  (mpcp-wallet-sdk session)
+    ↓  session.createSba({ amount, currency: "USDC", rail: "evm" })
+       → throws MpcpBudgetExceededError   if ceiling exceeded  ✗
+       → throws MpcpGrantRevokedError     if grant revoked     ✗
+       → returns SignedBudgetAuthorization                     ✓
+    ↓  x402 EIP-712 USDC payment on Base Sepolia
+Hyperbolic inference endpoint
+    ↓  LLM response
+Agent produces answer
+```
+
+---
+
+## Prerequisites
+
+- Node.js 22+
+- A funded Base Sepolia wallet (USDC testnet tokens)
+- A running `mpcp-policy-authority` (see [mpcp-policy-authority](https://github.com/mpcp-protocol/mpcp-policy-authority))
+- An EC P-256 PKCS8 PEM key for MPCP SBA signing
+
+### Generate an MPCP signing key
+
+```bash
+openssl genpkey -algorithm EC -pkeyopt ec_paramgen_curve:P-256 \
+  | openssl pkcs8 -topk8 -nocrypt -out sba-key.pem
+```
+
+---
+
+## Setup
+
+```bash
+# 1. Install dependencies
+npm install
+
+# 2. Configure
+cp .env.example .env
+# Edit .env with your wallet key, address, and signing key
+
+# 3. Issue a grant (one-time)
+npm run issue-grant
+
+# 4. Run the agent
+npm run run-agent "What is the capital of France?"
+```
+
+Or run both steps together:
+
+```bash
+npm start
+```
+
+---
+
+## Environment variables
+
+| Variable | Required | Description |
+|---|---|---|
+| `AGENT_PRIVATE_KEY` | ✓ | Base Sepolia wallet private key (`0x...`) |
+| `AGENT_ADDRESS` | ✓ | Agent EVM address |
+| `MPCP_SBA_SIGNING_KEY_PEM` | ✓ | PKCS8 PEM key for SBA signing |
+| `POLICY_AUTHORITY_URL` | — | PA URL (default: `http://localhost:3100`) |
+| `GRANT_FILE` | — | Grant JSON path (default: `./grant.json`) |
+| `HYPERBOLIC_URL` | — | Inference endpoint |
+| `HYPERBOLIC_MODEL` | — | Model name |
+| `RPC_URL` | — | Base Sepolia RPC (default: public endpoint) |
+
+---
+
+## Success criteria (Phase 1)
+
+- [x] Agent answers a question; `session.remaining()` decreases
+- [x] Budget exceeded → `MpcpBudgetExceededError` thrown before any x402 payment
+- [x] Operator calls `POST /revoke` → next `session.createSba()` throws `MpcpGrantRevokedError`
+- [x] Full run from grant issuance to answered question with no manual steps beyond `npm start`
+
+---
+
+## Project structure
+
+```
+src/
+  config.ts     — env loading and constants
+  wallet.ts     — MPCP session setup and budget tracking
+  x402.ts       — MPCP-aware x402 fetch wrapper
+  policy.ts     — PolicyGrant issuance via PA REST API
+  agent.ts      — inference agent (Vercel AI SDK + Hyperbolic)
+scripts/
+  issue-grant.ts  — CLI: issue a test grant and write to disk
+  run-agent.ts    — CLI: run the agent with a sample question
+```
+
+---
+
+## Roadmap
+
+See [ROADMAP.md](./ROADMAP.md) for Phase 2 (full gateway stack) and Phase 3 (Virtuals integration).