docs(build-on-base): add Gasless USDC Transfers (EIP-3009) reference

[osr21]

What

Adds references/gasless-usdc-transfers.md to the build-on-base skill, plus a Task Routing row in SKILL.md.

The reference covers the EIP-3009 transferWithAuthorization pattern for gasless USDC transfers: the authorization envelope, exact EIP-712 domain values (including the name mismatch trap — USDC is "USD Coin" but EURC is "EURC"), token addresses for Base and Base Sepolia, a relayer-side validation checklist (validBefore cap, BigInt parsing, rate limiting, surfacing revert reasons), and the smart-wallet caveat.

Why

The skill's existing references cover Base Account / Base Pay flows, but agents asked to implement "pay with USDC without ETH" for EOA wallets have no routing target — and this pattern has two failure modes that repeatedly burn builders and are invisible until runtime:

1. Wrong EIP-712 domain name → generic "invalid signature" revert with no hint the domain is the problem.
2. Smart-contract wallets can't sign EIP-3009 → the flow fails only at submission, after the user has signed. The reference instructs detecting contract accounts up front (getCode) and routing to batched calls + paymaster instead — and notes the same audit applies to EIP-2612 permit.

Both are documented from production experience running this exact pattern (BasePay's gasless relay on Base mainnet), not from spec reading.

Scope

Docs only — one new reference file, one table row in SKILL.md. No changes to existing references or plugins. Follows the frontmatter and cross-linking conventions of the existing reference files.

Companion runnable demo submitted to base/demos as apps/gasless-usdc-payments/.

[cb-heimdall]

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