docs: add Agent Substrate research note

[rkoster]

What is this note about?

A first-pass research note on Agent Substrate, its actor/worker lifecycle model, sandbox multiplexing, suspend/resume and snapshotting, Kubernetes components, and relevance to Cloud Foundry workload isolation and density. The note distinguishes the working counter demonstration from aspirational architecture.

Checklist

• My note lives in research/ and uses a lowercase kebab-case filename.
• It starts from research/TEMPLATE.md and has valid frontmatter (title, author, date, tags, status, sources).
• It has the four sections: Summary, Key findings, CF relevance, Open questions.
• I added relevant tags (see the suggested vocabulary in IDEATION.md).
• Sources are linked.
• It's on-topic for agentic workloads on Cloud Foundry.

[itsouvalas]

I think with 45 we opened up pandora's box about novelty. Given how close k8s-agent-sandbox is at 62, we could probably drop this below 70, which also keeps it off the top. Either way LGreatTM