feat: add fail-closed pre-fund policy hook

[AgentTanuki]

Why

session.fund() currently prepares a transaction without an optional counterparty/policy decision point. Wallet agents need a last-mile gate on the exact provider, chain, amount, and hydrated job.

What

• adds optional fundPolicy to AcpAgent.create
• evaluates it before every funding branch prepares a transaction
• requires explicit { allow: true }
• returns typed FundPolicyDeniedError with machine-readable evidence
• preserves current behavior when no policy is configured
• documents the hook and exports the types

Validation

• TypeScript build passes
• Node tests cover explicit allow, denial, missing decision, verifier error, and no-policy compatibility (4/4)

AI disclosure: implemented with significant Codex assistance and independently reviewed against the transaction ordering and compiled/tested locally.

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Note

Medium Risk
Touches the on-chain funding gate for all session.fund() paths; mistakes could block legitimate funds or use stale job data, though behavior is unchanged when no policy is set and snapshot pinning mitigates async races.

Overview
Adds an optional fundPolicy on AcpAgent.create so wallet agents can approve or block funding using the exact provider, chain, amount, and hydrated AcpJob before any fund transaction is prepared.

session.fund() now calls enforceFundPolicy first (fail-closed: only { allow: true } proceeds; denial, missing decision, or thrown errors abort with FundPolicyDeniedError). Funding branches use a fixed job snapshot taken at the start of fund() so a slow policy or concurrent fetchJob() cannot change the job the policy saw. Omitting fundPolicy leaves prior behavior unchanged.

New src/core/fundPolicy.ts types and helper are exported from the package; README documents the hook. npm test runs compile plus Node tests for allow/deny, missing decision, errors, and snapshot stability.

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