Add SDK safe logging guidelines

[silentgeckoaudit3801]

Fixes #131.

Adds safe logging guidance for SDK contributors and consumers, covering:

• values that must never be logged, including secret keys, raw signed transactions, full transaction payloads, auth/session values, and unsafe error objects
• safe redaction patterns for accounts, transaction hashes, and errors
• unsafe vs. safe debug logging examples
• debug-mode expectations for local development and production builds

Validation: static documentation/API review only; no project dependencies or toolchains were run from this environment.