docs: add Boundary agent workloads research note

[rkoster]

What is this note about?

This note researches HashiCorp Boundary as an identity-based access broker for agent workloads. It covers session-scoped target access, workers, credential brokering, lifecycle and audit considerations, and how Boundary might complement Cloud Foundry without replacing workload sandboxing.

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.

Test Plan

• Fallback structural and whitespace checks passed.
• Python validator, rating check, and unit tests could not run locally because python3 is unavailable; CI should run the repository's full checks.