docs: add Docker Sandboxes research note

[rkoster]

Closes #64

What is this note about?

A research note on Docker Sandboxes as local microVM isolation for coding agents. It distinguishes its local sbx-managed harness model from a remote, multi-tenant sandbox execution API, and compares that boundary with OpenSandbox lifecycle and execution APIs. It covers workspace modes, Docker isolation, network and credential mediation, MCP policy, and Cloud Foundry relevance.

Checklist

• My note lives in research/ and uses a lowercase kebab-case filename.
• It starts from research/TEMPLATE.md and has valid frontmatter.
• It has the four required sections.
• I added relevant tags.
• Sources are linked.
• It is on-topic for agentic workloads on Cloud Foundry.

[wayneeseguin]

Let's merge it.