Why: Record the approved design for the interaction foundation phase — inbox/
command-bus/workspace mechanisms sink into the base system, domain views stay
plugin territory (target-system strategy), decisions D25-D33.
Evidence: proposal covers background, five UI principles, dialogue-completeness
P0 constraint, module/repo split (Agent-System-UI, Agent-Command), contract
sketch, task sequence T0-T5 and acceptance criteria.
Impact: Becomes the design baseline for T0-T5 tasks across five repos.
Refs: Agent-System-Dev feature/ui-foundation-r0
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