feat: add AgentsKit Atlas vector memory example

[EmersonBraun]

What this adds

• a small TypeScript example that connects the official MongoDB driver to AgentsKit's Atlas vector-memory adapter
• a complete store, $vectorSearch, retrieval, bounded indexing retry, and cleanup lifecycle
• deterministic three-dimensional fixtures, an Atlas index definition, environment setup, and credential-free tests
• an entry in the showcase app index

Why

Atlas already provides the retrieval layer. This example shows how to keep that layer independent from the agent runtime, model, and embedding provider instead of rebuilding the memory contract when one component changes. Atlas Vector Search indexes new records asynchronously, so the lifecycle retries search for a bounded period and always cleans up in finally.

Validation

• npm run typecheck
• npm test — 3/3 passed, including immediate visibility, delayed visibility, and retry timeout
• npm audit --audit-level=high — 0 vulnerabilities
• repository pre-commit hooks passed for every changed file
• git diff --check
• live Atlas M0 validation: store, vector search, and cleanup passed against a disposable project and three-dimensional cosine index
• GitGuardian and Semgrep checks passed

The live project, cluster, credentials, network rule, index, and synthetic records were removed after validation.