fix(fund): drop broken Coinbase Onramp URL builder

[vvillait88]

Summary

The URL agentscore-pay fund printed for Base + Solana mainnet was broken end-to-end:

1. Coinbase rejected the legacy URL flow with "Missing or invalid parameters / appId required".
2. Once an appId is added, the project's "secure initialization" setting still rejects it — Coinbase steers integrators toward server-minted sessionTokens instead.

Stopgap

Drop the auto-onramp link entirely. fund now prints the receive QR + polls balance for every mainnet chain (matching existing Tempo UX). Users send USDC from any source — another wallet, a CEX withdrawal, or any third-party fiat onramp that supports the destination chain.

Tempo testnet's programmatic mint via tempo_fundAddress is unchanged. Base Sepolia + Solana devnet still use the existing faucet command.

Future direction

Stripe Crypto Onramp via a server-minted session endpoint (pending Stripe approval). Same architectural pattern (backend mints sessionToken, CLI opens the hosted URL) applies to both Coinbase Onramp Session Token API and Stripe Crypto Onramp — one server-side onramp-broker endpoint will support both providers when we reach that.

Changes

• Remove onrampUrl() function from constants.ts
• Drop onramp_url field from FundResult + CreateResult
• Update CLI descriptions/hints/examples + agent-guide notes
• Rewrite README funding section
• Update internal CLAUDE.md
• Remove broken onrampUrl tests

Test plan

• 352/352 vitest pass against synced ed25519-hd-key@2.0.0 (was 356/356; 4 tests removed alongside the deleted onrampUrl function)
• tsc --noEmit clean
• bun install produces no diff (lockfile in sync)
• Mintlify docs paired in agentscore/core#TBD

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