docs: Add redaction and allowlist documentation to README

[Copilot]

The README had no documentation on automatic secret redaction, how to configure an allowlist, or how to use REDACT_CONFIG. This adds a complete Redacting Sensitive Data section covering all of that.

What's documented

• Auto-redacted patterns — hex, Base64, Base64url, Base58, BIP39 mnemonics → [REDACTED]
• setupLogging with RedactConfig — pass allow rules directly at startup
• Allowlist / context rules — shape of a rule (re, before, after), how context slicing works, and examples showing which values get kept vs. redacted
• REDACT_CONFIG env var — step-by-step: build JSON → Base64-encode → set env var; clarifies that both sources are merged

Example

// Suppress redaction for known-safe hex contexts
setupLogging({
  hex: { allow: [{ re: /\b(event|transfer)\b/i }] },
});

// Via environment (Base64-encoded JSON):
// {"hex":{"allow":[{"re":"\\b(event)\\b","flags":"i"}]}}
export REDACT_CONFIG="eyJoZXgiOnsiYWxsb3ciOlt7..."

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