docs: Add BEP-1043 for fixing multi agent device allocation

[hhoikoo]

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation
• Documentation
  • Contents in the docs directory
  • docstrings in public interfaces and type annotations

[Copilot]

Pull request overview

Adds a new Backend.AI Enhancement Proposal (BEP) documenting a design to change multi-agent resource allocation from slot-based partitioning to explicit device-based assignments.

Changes:

• Register BEP-1041 in the proposals index.
• Add a new BEP document describing motivation, proposed architecture (device partitioners, distribution strategy), migration considerations, and an implementation plan.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
proposals/README.md	Adds BEP-1041 to the BEP registry table for discoverability.
proposals/BEP-1041-multi-agent-device-split.md	Introduces the BEP-1041 proposal detailing device-based allocation for multi-agent scenarios.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[hhoikoo]

Superceded by #8501