feat(logger): serverless-safe pretty logger + err serializers

[priosshrsth]

Summary

• Add err/error serializers to the default logger so Error objects log message + stack instead of {} in serverless runtimes (Vercel/Cloudflare).
• Add createPrettyLogger() — runs pino-pretty as a synchronous stream (no worker thread), so it's serverless-safe. Dynamically imported and declared as an optional peer dependency, so plain-JSON consumers never bundle it.
• Docs updated (README, root + contributor CLAUDE.md).

Usage

import { createLogger, createPrettyLogger } from "assembly-kit/logger";

const log =
  process.env.NODE_ENV === "development"
    ? await createPrettyLogger()
    : createLogger();

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