feat(examples): Add MoonBit client example for PowerMem HTTP API

[tonyfettes]

Summary

This PR adds a simple, lightweight MoonBit client example demonstrating how to integrate PowerMem's intelligent memory capabilities into MoonBit applications.

Solution Description

It mirrors the structure of examples/go/ — a self-contained, runnable demo with an inline HTTP client, inline request/response types, and a single main that walks through the core API surface. No dependencies beyond moonbitlang/core and moonbitlang/async (MoonBit's stdlib-tier packages), so the directory is copy-out runnable.

Uses the same POWERMEM_BASE_URL / POWERMEM_API_KEY environment variables as the Go example.

Tested against a local server (powermem-server --host 0.0.0.0 --port 8000, SQLite + Ollama.

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[Teingi]

LGTM