docs(mcp): clarify SIWX reuse extension path

[Bortlesboat]

Description

This docs-only PR connects the existing MCP and Sign-In-With-X examples so readers can understand the current path to "pay once, then reopen the same paid MCP tool without repaying."

It does three things:

• clarifies that the current MCP example is intentionally payment-first
• documents that @x402/mcp createPaymentWrapper() still requires payment requirements and does not yet provide a first-class auth-only MCP helper
• cross-links the MCP and SIWX examples so the wallet-auth building blocks are easier to find

This is based on a locally verified MCP + SIWX operator flow where the first MCP tool call settles normally and later calls to that same exact tool are reopened through wallet-auth plus stored entitlement.

Tests

• git diff --check
• Manual review of the three touched README files
• No runtime tests run; this is a docs-only change

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits
• I added a changelog fragment for user-facing changes (docs-only changes can skip)

[Bortlesboat]

Reopened this against the canonical repo at x402-foundation#1939 because coinbase/x402 is currently a fork of x402-foundation/x402 and is behind Foundation main. Closing this fork-targeted copy to keep review in one place.