docs: document Base Sepolia account readiness limitation

[osr21]

Summary

• document the currently reported Base Sepolia limitation for newly created Base Accounts beside the testnet: true Base Pay example
• clarify that capability discovery does not guarantee that the connected account has completed hosted-wallet provisioning on that chain
• point developers to Bug: Base Sepolia transactions refused for newly-created 7702 accounts, while wallet_getCapabilities reports the chain as supported #363 and provide non-speculative integration-testing guidance

This is intentionally documentation-only because the rejection occurs in the hosted wallet flow rather than in this SDK repository.

Documents #363 while the underlying hosted-wallet behavior is investigated.

How did you test your changes?

• reviewed both rendered Markdown insertion points
• confirmed the change only modifies README.md and packages/account-sdk/README.md
• checked the patch for whitespace errors

[cb-heimdall]

🟡 Heimdall Review Status

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