docs(examples): clarify facilitator signer roles

[Bortlesboat]

This small docs/examples cleanup aligns the basic facilitator READMEs around the operator guidance we hit while running a real self-hosted Base Sepolia lane locally.

What changed

• clarify that the facilitator signer should be a dedicated funded key
• call out that it should stay separate from seller payTo wallets and buyer test wallets
• make the Python and TypeScript examples match the stronger Go guidance

Why

When we ran the self-hosted facilitator example in practice, the easiest operator mistake was reusing the buyer wallet or thinking the facilitator key was interchangeable with the seller identity wallet. This keeps the example docs closer to real operator setup.

Validation

• README-only change
• git diff --check clean

[Bortlesboat]

Reopened this against the canonical repo at x402-foundation#1937 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.