Agent re-architecture design

Re-architect the Backend.AI Agent. This is a separate design line from BA-6074 (Agent Kernel Lifecycle Structuring). Establish a target architecture for the agent's structure and rewritability before implementation begins.

The concrete design is produced via a BEP first (see the child 'Write BEP' task), then decomposed into implementation stories via /bep-to-issues.

JIRA Issue: BA-6684

Start with the child “Write BEP” task and review how this design line differs from BA-6074, Agent Kernel Lifecycle Structuring. Produce the target architecture for the agent’s structure and rewritability, then use the resulting BEP to decompose the work into implementation stories via /bep-to-issues.

Written by the indexing model from the issue text.