docs(examples): add x402ActionProvider pre-execution safety guard example

[teodorofodocrispin-cmyk]

What this adds

A minimal, standalone example demonstrating a pattern: gating an on-chain transaction behind a third-party x402 safety check, using the existing x402ActionProvider -- no new dependencies added to the framework itself.

The pattern

1. Build the candidate transaction (to, value, data).
2. Call an x402-compatible guard/safety service via makeHttpRequestWithX402 (pays the small USDC fee automatically).
3. Only proceed to sign/execute if the verdict is safe.

This is deliberately not wired through the LLM tool-calling loop for the safety decision itself -- the check should run every time, deterministically, regardless of what the model decides. The example calls the action provider's methods directly in code.

About the third-party service used in the example

The example uses SENTINEL as the guard service purely because its response (verdict / risks / score) is easy to branch on for a clear example. It is independent and unaffiliated with Coinbase/AgentKit -- I'm one of its maintainers, flagging that plainly. The pattern generalizes to any x402-compatible guard service; swapping GUARD_URL and the response-parsing is enough. Happy to genericize this further (e.g. parameterize the guard URL/schema) if that's preferred to naming one service in an official example.

Verification

• TypeScript syntax/type-checks cleanly in isolation (tsc --noEmit --skipLibCheck) -- no errors beyond expected unresolved workspace (@coinbase/agentkit) and external (dotenv) packages, which I don't have installed outside the monorepo.
• I was not able to run this against the full monorepo build/lint/test pipeline from my environment. Structure (package.json, tsconfig.json, eslint/prettier configs) mirrors typescript/examples/langchain-cdp-chatbot as closely as possible for consistency, but please flag anything that doesn't match current conventions -- glad to adjust.

Files added

typescript/examples/x402-sentinel-preflight-guard/

• guard-before-transfer.ts -- the example itself
• README.md
• package.json, tsconfig.json, .eslintrc.json, .prettierrc, .prettierignore, .env-local

[cb-heimdall]

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